Implement isolated event loop for spawnSync (#24436)

CLAUDE.md

@@ -38,16 +38,36 @@ If no valid issue number is provided, find the best existing file to modify inst
 
 ### Writing Tests
 
-Tests use Bun's Jest-compatible test runner with proper test fixtures:
+Tests use Bun's Jest-compatible test runner with proper test fixtures.
+
+- For **single-file tests**, prefer `-e` over `tempDir`.
+- For **multi-file tests**, prefer `tempDir` and `Bun.spawn`.
 
 ```typescript
 import { test, expect } from "bun:test";
 import { bunEnv, bunExe, normalizeBunSnapshot, tempDir } from "harness";
 
-test("my feature", async () => {
+test("(single-file test) my feature", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "console.log('Hello, world!')"],
+    env: bunEnv,
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([
+    proc.stdout.text(),
+    proc.stderr.text(),
+    proc.exited,
+  ]);
+
+  expect(normalizeBunSnapshot(stdout)).toMatchInlineSnapshot(`"Hello, world!"`);
+  expect(exitCode).toBe(0);
+});
+
+test("(multi-file test) my feature", async () => {
   // Create temp directory with test files
   using dir = tempDir("test-prefix", {
-    "index.js": `console.log("hello");`,
+    "index.js": `import { foo } from "./foo.ts"; foo();`,
+    "foo.ts": `export function foo() { console.log("foo"); }`,
   });
 
   // Spawn Bun process