Test: Prep require-qunit.js test for ESM package

Extracted from #1798 to minimize
the diff.

src/cli/require-qunit.js

@@ -38,8 +38,8 @@ module.exports = function requireQUnit (resolve = require.resolve) {
       //   If you get "Error: Cannot find module ../../qunit/qunit", it probably
       //   means you haven't yet run "npm run build".
       //
-      delete require.cache[resolve('../../qunit/qunit')];
-      return require('../../qunit/qunit');
+      delete require.cache[resolve('../../qunit/qunit.js')];
+      return require('../../qunit/qunit.js');
     }
 
     throw e;

test/cli/require-qunit-test.js

@@ -1,41 +1,69 @@
-const proxyquire = require('proxyquire').noCallThru();
+const fs = require('fs');
+const os = require('os');
 const path = require('path');
-const resolveStub = (path) => {
+
+const proxyquire = require('proxyquire').noCallThru();
+
+function resolveStub (path) {
   return path;
-};
+}
 
 QUnit.module('requireQUnit', function (hooks) {
   const cwd = process.cwd();
+  const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'qunit-test-'));
 
   hooks.before(() => {
     process.chdir(cwd);
   });
   hooks.afterEach(() => {
     process.chdir(cwd);
+    fs.rmSync(tmpDir, { force: true, recursive: true });
   });
 
-  QUnit.test('finds QUnit package in the current working directory', function (assert) {
-    const requireQUnit = require('../../src/cli/require-qunit');
-    process.chdir(path.join(__dirname, './fixtures/require-from-cwd'));
+  QUnit.test('find local qunit in the current working directory', function (assert) {
+    // We can't use a fixture inside the project directory (e.g. /test/cli/fixtures)
+    // because Node.js will always find 'qunit' from inside our own package, no
+    // matter what paths you pass to require.resolve() or even Module.createRequire(),
+    // it will always look for LOAD_PACKAGE_SELF first [1], and return from there if
+    // it has "exports" set in package.json.
+    //
+    // [1]: https://nodejs.org/docs/latest-v18.x/api/modules.html#all-together from inside the a  inside a project that uses "exports" in package.json
+    //
+    // create:
+    // - tmpDir/
+    // - tmpDir/node_modules/
+    // - tmpDir/node_modules/qunit/
+    // - tmpDir/node_modules/qunit/index.js
+    fs.mkdirSync(
+      path.join(tmpDir, 'node_modules', 'qunit'),
+      { recursive: true }
+    );
+    fs.writeFileSync(
+      path.join(tmpDir, 'node_modules', 'qunit', 'index.js'),
+      'module.exports = { version: "from-cwd" };'
+    );
+    const requireQUnit = require('../../src/cli/require-qunit.js');
+
+    process.chdir(tmpDir);
 
     assert.strictEqual(requireQUnit().version, 'from-cwd');
   });
 
   // For development mode invoked locally,
   // or for global install without local dependency installed.
-  QUnit.test('finds relative self', function (assert) {
+  QUnit.test('find relative self', function (assert) {
     const fakeQUnit = {
       version: 'from-fake'
     };
-    const requireQUnit = proxyquire('../../src/cli/require-qunit', {
+    const requireQUnit = proxyquire('../../src/cli/require-qunit.js', {
       qunit: null,
       '../../qunit/qunit': fakeQUnit
     });
     assert.strictEqual(requireQUnit(resolveStub).version, 'from-fake');
   });
 
-  QUnit.test('throws error if none of the modules are found', function (assert) {
-    const requireQUnit = proxyquire('../../src/cli/require-qunit', {
+  QUnit.test('throw if none found', function (assert) {
+    const requireQUnit = proxyquire('../../src/cli/require-qunit.js', {
       qunit: null,
       '../../qunit/qunit': null
     });