test(async): fix flaky waitFor test (#6467)

Author: kt3k

async/unstable_wait_for_test.ts

@@ -1,27 +1,40 @@
 // Copyright 2018-2025 the Deno authors. MIT license.
 import { assertEquals, assertLess, assertRejects } from "@std/assert";
 import { waitFor } from "./unstable_wait_for.ts";
+import { FakeTime } from "@std/testing/time";
 
 // NOT detecting leaks means that the internal interval was correctly cleared
 
 Deno.test("waitFor() returns fulfilled promise", async () => {
+  using time = new FakeTime();
+
   let flag = false;
-  setTimeout(() => flag = true, 100);
-  const start = performance.now();
-  await waitFor(() => flag === true, 1000);
-  assertLess(performance.now() - start, 1000);
+  setTimeout(() => {
+    flag = true;
+  }, 100);
+  const startedAt = Date.now();
+  const promise = waitFor(() => flag, 1000);
+  time.tick(500);
+  await promise;
+  assertLess(Date.now() - startedAt, 1000);
 });
 
 Deno.test("waitFor() throws DOMException on timeout", async () => {
+  using time = new FakeTime();
+
   let flag = false;
-  const id = setTimeout(() => flag = true, 1000);
-  const start = performance.now();
-  const error = await assertRejects(
-    () => waitFor(() => flag === true, 100),
+  const id = setTimeout(() => {
+    flag = true;
+  }, 1000);
+  const start = Date.now();
+  const assertion = assertRejects(
+    () => waitFor(() => flag, 100),
     DOMException,
     "Signal timed out.",
   );
-  assertLess(performance.now() - start, 1000);
+  time.tick(500);
+  const error = await assertion;
+  assertLess(Date.now() - start, 1000);
   assertEquals(error.name, "TimeoutError");
   clearTimeout(id);
 });