Add regression test for Jest --findRelatedTests behavior

- a lint rule might be better long-term, but this will at least warn us if our hook starts running extra tests again

Author: cee-chen

scripts/tests/test-staged.test.js

@@ -0,0 +1,24 @@
+const { execSync } = require('child_process');
+const path = require('path');
+const euiRoot = path.resolve(__dirname, '../../');
+
+describe('test-staged.js', () => {
+  it('does not have --findRelatedTests regressions due to imports from src/components', () => {
+    const outputString = execSync(
+      'yarn test-unit --findRelatedTests --listTests src/components/beacon/beacon.tsx',
+      { cwd: euiRoot }
+    ).toString();
+    const output = outputString
+      .split('\n')
+      .filter((item) => item.includes('eui/src/components'));
+
+    expect(
+      output[0].endsWith('eui/src/components/tour/tour_step.test.tsx')
+    ).toBeTruthy();
+    expect(
+      output[1].endsWith('eui/src/components/beacon/beacon.test.tsx')
+    ).toBeTruthy();
+
+    expect(output[2]).toBeUndefined();
+  });
+});