add missing test file

test/fake-timers-esm-test.js

@@ -0,0 +1,21 @@
+import * as timers from '../pkg/fake-timers-esm.js';
+
+const expectedExports = ['timers', 'createClock', 'install', 'withGlobal'];
+
+for (const exp of expectedExports) {
+    if (!timers[exp]) {
+        console.error(`Missing export in ES module: ${exp}`);
+        process.exit(1);
+    }
+}
+
+let hasRun = false;
+const org = setTimeout;
+const clock = timers.install();
+setTimeout(() => (hasRun = true));
+clock.tick();
+
+if (!hasRun) {
+    console.error('Failed to tick timers in ES Module');
+    process.exit(1);
+}