message: Switch to WaitForNotification to fix the flaky test.
  See:
diff: @@ -487,8 +487,7 @@ TEST(RamFileBlockCacheTest, CoalesceConcurrentReads) {
  TF_EXPECT_OK(ReadCache(&cache, "", 0, block_size / 2, &out));
  EXPECT_EQ(out.size(), block_size / 2);
  }));
  - EXPECT_TRUE(WaitForNotificationWithTimeout(&notification, 10000))
  - << "Timeout waiting for concurrent thread to start.";
  + notification.WaitForNotification();
  std::vector<char> out;
  TF_EXPECT_OK(ReadCache(&cache, "", block_size / 2, block_size / 2, &out));
  EXPECT_EQ(out.size(), block_size / 2);
  