fix core: fix flaky ThreadLocal.SafeThreadLocalWorks test

This test was copied from `TaskUsesCorrectInstanceAfterSleep` test. Together with the copying, all `Sleep` times were decreased by 10 times. But one `Sleep` time was left as `3s` instead of `300ms`, which broke the test logic and made the test very slightly flaky.
e69bc71a4fada77c7f4bbaed3773e852e7b42699

Pull Request resolved: #583

core/src/engine/thread_local_test.cpp

@@ -151,7 +151,7 @@ UTEST_MT(ThreadLocal, SafeThreadLocalWorks, 2) {
     return SafeLoadThreadLocal();
   });
 
-  engine::SleepFor(3s);
+  engine::SleepFor(300ms);
 
   // (6)
   EXPECT_NE(pthread_self(), thread1_id);