New: add new rule no-only-tests (#145)

Most other test frameworks have a similar rule to this:
* [ava/no-only-test](https://github.com/avajs/eslint-plugin-ava/blob/main/docs/rules/no-only-test.md)
* [jest/no-focused-tests](https://github.com/jest-community/eslint-plugin-jest/blob/main/docs/rules/no-focused-tests.md)
* [mocha/no-exclusive-tests](https://github.com/lo1tuma/eslint-plugin-mocha/blob/master/docs/rules/no-exclusive-tests.md)
* [qunit/no-only](https://github.com/platinumazure/eslint-plugin-qunit/blob/master/docs/rules/no-only.md)

README.md

@@ -53,6 +53,7 @@ Name | ✔️ | 🛠 | Description
 [no-deprecated-report-api](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-deprecated-report-api.md) | ✔️ | 🛠 | disallow use of the deprecated context.report() API
 [no-identical-tests](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-identical-tests.md) | ✔️ | 🛠 | disallow identical tests
 [no-missing-placeholders](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-missing-placeholders.md) | ✔️ |  | disallow missing placeholders in rule report messages
+[no-only-tests](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-only-tests.md) |  |  | disallow the test case property `only`
 [no-unused-placeholders](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-unused-placeholders.md) | ✔️ |  | disallow unused placeholders in rule report messages
 [no-useless-token-range](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-useless-token-range.md) | ✔️ | 🛠 | disallow unnecessary calls to sourceCode.getFirstToken and sourceCode.getLastToken
 [prefer-object-rule](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/prefer-object-rule.md) |  | 🛠 | disallow rule exports where the export is a function.

docs/rules/no-only-tests.md

@@ -0,0 +1,55 @@
+# Disallow the test case property `only` (no-only-tests)
+
+The [`only` property](https://eslint.org/docs/developer-guide/unit-tests#running-individual-tests) can be used as of [ESLint 7.29](https://eslint.org/blog/2021/06/eslint-v7.29.0-released#highlights) for running individual rule test cases with less-noisy debugging. This feature should be only used in development, as it prevents all the tests from running. Mistakenly checking-in a test case with this property can cause CI tests to incorrectly pass.
+
+## Rule Details
+
+This rule flags a violation when a test case is using `only`. Note that this rule is not autofixable since automatically deleting the property would prevent developers from being able to use it during development.
+
+Examples of **incorrect** code for this rule:
+
+```js
+/* eslint eslint-plugin/no-only-tests: error */
+
+const { RuleTester } = require('eslint');
+const ruleTester = new RuleTester();
+
+ruleTester.run('my-rule', myRule, {
+  valid: [
+    {
+      code: 'const valid = 42;',
+      only: true,
+    },
+    RuleTester.only('const valid = 42;'),
+  ],
+  invalid: [
+    {
+      code: 'const invalid = 42;',
+      only: true,
+      errors: [/* ... */],
+    },
+  ],
+});
+```
+
+Examples of **correct** code for this rule:
+
+```js
+/* eslint eslint-plugin/no-only-tests: error */
+
+const { RuleTester } = require('eslint');
+const ruleTester = new RuleTester();
+
+ruleTester.run('my-rule', myRule, {
+  valid: [
+    'const valid = 42;',
+    { code: 'const valid = 42;' },
+  ],
+  invalid: [
+    {
+      code: 'const invalid = 42;',
+      errors: [/* ... */],
+    },
+  ],
+});
+```

lib/rules/no-only-tests.js

@@ -0,0 +1,112 @@
+'use strict';
+
+const utils = require('../utils');
+
+/**
+ * Checks if the given token is a comma token or not.
+ * From: https://github.com/eslint/eslint/blob/master/lib/rules/utils/ast-utils.js
+ * @param {Token} token The token to check.
+ * @returns {boolean} `true` if the token is a comma token.
+ */
+function isCommaToken (token) {
+  return token.value === ',' && token.type === 'Punctuator';
+}
+
+/**
+ * Checks if the given token is an opening brace token or not.
+ * From: https://github.com/eslint/eslint/blob/master/lib/rules/utils/ast-utils.js
+ * @param {Token} token The token to check.
+ * @returns {boolean} `true` if the token is an opening brace token.
+ */
+function isOpeningBraceToken (token) {
+  return token.value === '{' && token.type === 'Punctuator';
+}
+
+/**
+ * Checks if the given token is a closing brace token or not.
+ * From: https://github.com/eslint/eslint/blob/master/lib/rules/utils/ast-utils.js
+ * @param {Token} token The token to check.
+ * @returns {boolean} `true` if the token is a closing brace token.
+ */
+function isClosingBraceToken (token) {
+  return token.value === '}' && token.type === 'Punctuator';
+}
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow the test case property `only`',
+      category: 'Tests',
+      recommended: false,
+    },
+    schema: [],
+    messages: {
+      foundOnly:
+        'The test case property `only` can be used during development, but should not be checked-in, since it prevents all the tests from running.',
+      removeOnly: 'Remove `only`.',
+    },
+    hasSuggestions: true,
+  },
+
+  create (context) {
+    return {
+      Program (ast) {
+        for (const testRun of utils.getTestInfo(context, ast)) {
+          for (const test of [...testRun.valid, ...testRun.invalid]) {
+            if (test.type === 'ObjectExpression') {
+              // Test case object: { code: 'const x = 123;', ... }
+
+              const onlyProperty = test.properties.find(
+                property =>
+                  property.key.type === 'Identifier' &&
+                  property.key.name === 'only' &&
+                  property.value.type === 'Literal' &&
+                  property.value.value
+              );
+
+              if (onlyProperty) {
+                context.report({
+                  node: onlyProperty,
+                  messageId: 'foundOnly',
+                  suggest: [
+                    {
+                      messageId: 'removeOnly',
+                      *fix (fixer) {
+                        const sourceCode = context.getSourceCode();
+
+                        const tokenBefore = sourceCode.getTokenBefore(onlyProperty);
+                        const tokenAfter = sourceCode.getTokenAfter(onlyProperty);
+                        if (
+                          (isCommaToken(tokenBefore) && isCommaToken(tokenAfter)) || // In middle of properties
+                          (isOpeningBraceToken(tokenBefore) && isCommaToken(tokenAfter)) // At beginning of properties
+                        ) {
+                          yield fixer.remove(tokenAfter); // Remove extra comma.
+                        }
+                        if (isCommaToken(tokenBefore) && isClosingBraceToken(tokenAfter)) { // At end of properties
+                          yield fixer.remove(tokenBefore); // Remove extra comma.
+                        }
+
+                        yield fixer.remove(onlyProperty);
+                      },
+                    },
+                  ],
+                });
+              }
+            } else if (
+              test.type === 'CallExpression' &&
+              test.callee.type === 'MemberExpression' &&
+              test.callee.object.type === 'Identifier' &&
+              test.callee.object.name === 'RuleTester' &&
+              test.callee.property.type === 'Identifier' &&
+              test.callee.property.name === 'only'
+            ) {
+              // RuleTester.only('const x = 123;');
+              context.report({ node: test.callee, messageId: 'foundOnly' });
+            }
+          }
+        }
+      },
+    };
+  },
+};