[fixit] Make activity_test lighter weight (#30575)

* [activity] Make test lighter weight

* Automated change: Fix sanity tests

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

test/core/promise/activity_test.cc

@@ -360,25 +360,28 @@ TEST(AtomicWakerTest, ThreadStress) {
   AtomicWaker waker;
 
   threads.reserve(90);
-  for (int i = 0; i < 30; i++) {
+  for (int i = 0; i < 5; i++) {
     threads.emplace_back([&] {
       while (!done.load(std::memory_order_relaxed)) {
         waker.Wakeup();
+        std::this_thread::sleep_for(std::chrono::milliseconds(1));
       }
     });
   }
-  for (int i = 0; i < 30; i++) {
+  for (int i = 0; i < 5; i++) {
     threads.emplace_back([&] {
       while (!done.load(std::memory_order_relaxed)) {
         waker.Set(Waker(new TestWakeable(&wakeups, &drops)));
+        std::this_thread::sleep_for(std::chrono::milliseconds(1));
       }
     });
   }
-  for (int i = 0; i < 30; i++) {
+  for (int i = 0; i < 5; i++) {
     threads.emplace_back([&] {
       while (!done.load(std::memory_order_relaxed)) {
         (waker.Armed() ? &armed : &not_armed)
             ->fetch_add(1, std::memory_order_relaxed);
+        std::this_thread::sleep_for(std::chrono::milliseconds(1));
       }
     });
   }