[Atomics.waitAsync] Mock Atomics.waitAsync in correctness fuzzing

It's inherently timing-dependent, leading to false positives.

Bug: chromium:1127612, v8:10239

Change-Id: Ibf6f3cb07f520a404daf8e860a0c2e5f2319529e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2423707
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70046}

test/mjsunit/tools/foozzie.js

@@ -107,3 +107,8 @@ function callPow(v) {
 const unoptimized = callPow(6996);
 %OptimizeFunctionOnNextCall(callPow);
 assertEquals(unoptimized, callPow(6996));
+
+// Test mocked Atomics.waitAsync.
+let then_called = false;
+Atomics.waitAsync().value.then(() => {then_called = true;});
+assertEquals(true, then_called);

tools/clusterfuzz/v8_mock.js

@@ -197,3 +197,10 @@ FinalizationRegistry.prototype.register = function(target, holdings) { };
 FinalizationRegistry.prototype.unregister = function(unregisterToken) { };
 FinalizationRegistry.prototype.cleanupSome = function() { };
 FinalizationRegistry.prototype[Symbol.toStringTag] = "FinalizationRegistry";
+
+// Mock the nondeterministic Atomics.waitAsync.
+Atomics.waitAsync = function() {
+  // Return a mock "Promise" whose "then" function will call the callback
+  // immediately.
+  return {'value': {'then': function (f) { f(); }}};
+}