Add tests for filename with lowercase, uppercase, slashes, and spaces

tests/fixtures/filename.md

@@ -0,0 +1,5 @@
+# Test
+
+```js filename="a/b/C D E.js"
+console.log("...");
+```

tests/plugin.test.js

@@ -1436,6 +1436,21 @@ describe("FlatESLint", () => {
 			assert.strictEqual(results[0].messages[4].column, 2);
 		});
 
+		it("parses when file name includes lowercase, uppercase, slashes, and spaces", async () => {
+			const results = await eslint.lintFiles([
+				path.resolve(__dirname, "./fixtures/filename.md"),
+			]);
+
+			assert.strictEqual(results.length, 1);
+			assert.strictEqual(results[0].messages.length, 1);
+			assert.strictEqual(
+				results[0].messages[0].message,
+				"Unexpected console statement.",
+			);
+			assert.strictEqual(results[0].messages[0].line, 4);
+			assert.strictEqual(results[0].messages[0].column, 1);
+		});
+
 		// https://github.com/eslint/markdown/issues/181
 		it("should work when called on nested code blocks in the same file", async () => {
 			/*

tests/processor.test.js

@@ -401,6 +401,20 @@ describe("processor", () => {
 					assert.strictEqual(blocks[0].filename, "abc/def.js");
 				});
 
+				it("should parse a filename with lowercase, uppercase, slashes, and spaces", () => {
+					const code =
+						prefix +
+						[
+							"```  js filename='a/b/C D E.js'",
+							"var answer = 6 * 7;",
+							"```",
+						].join("\n");
+					const blocks = processor.preprocess(code);
+
+					assert.strictEqual(blocks.length, 1);
+					assert.strictEqual(blocks[0].filename, "a/b/C D E.js");
+				});
+
 				it("should parse a filename each from two meta", () => {
 					const code =
 						prefix +