test: ensure microtask queues are not automatically drained

V8 had a bug that in certain cases the default microtask queue was
drained, and this test relies on the behavior.

Backport-PR-URL: #39470
PR-URL: #38990
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/cctest/test_environment.cc

@@ -647,7 +647,8 @@ TEST_F(EnvironmentTest, NestedMicrotaskQueue) {
   const v8::HandleScope handle_scope(isolate_);
   const Argv argv;
 
-  std::unique_ptr<v8::MicrotaskQueue> queue = v8::MicrotaskQueue::New(isolate_);
+  std::unique_ptr<v8::MicrotaskQueue> queue = v8::MicrotaskQueue::New(
+      isolate_, v8::MicrotasksPolicy::kExplicit);
   v8::Local<v8::Context> context = v8::Context::New(
       isolate_, nullptr, {}, {}, {}, queue.get());
   node::InitializeContext(context);