message: core: fix macOS CI
  There is so much jitter in macOS CI on travis that we need to slow down
  the thead pool test, yet again.
diff: @@ -17,7 +17,7 @@ static Time our_time;
  static void run_delayed()
  {
  - our_time.sleep_for(std::chrono::milliseconds(50));
  + our_time.sleep_for(std::chrono::milliseconds(250));
  task_one_ran = true;
  }
  @@ -29,9 +29,9 @@ TEST(ThreadPool, SimpleTask)
  task_one_ran = false;
  tp.enqueue(std::bind(run_delayed));
  - our_time.sleep_for(std::chrono::milliseconds(25));
  + our_time.sleep_for(std::chrono::milliseconds(125));
  EXPECT_FALSE(task_one_ran);
  - our_time.sleep_for(std::chrono::milliseconds(50));
  + our_time.sleep_for(std::chrono::milliseconds(250));
  EXPECT_TRUE(task_one_ran);
  }
  @@ -51,7 +51,7 @@ TEST(ThreadPool, SimpleTaskWithArgs)
  tp.enqueue(std::bind(add_first_to_second, first, std::ref(second)));
  - our_time.sleep_for(std::chrono::milliseconds(25));
  + our_time.sleep_for(std::chrono::milliseconds(125));
  EXPECT_EQ(second, sum);
  }
  @@ -66,7 +66,7 @@ TEST(ThreadPool, LambdaWithArgs)
  tp.enqueue([first, &second]() { second += first; });
  - our_time.sleep_for(std::chrono::milliseconds(25));
  + our_time.sleep_for(std::chrono::milliseconds(125));
  EXPECT_EQ(second, sum);
  }
  @@ -82,7 +82,7 @@ TEST(ThreadPool, ManyTasks)
  tp.enqueue([&tasks, i]() { tasks[i] = i; });
  }
  - our_time.sleep_for(std::chrono::milliseconds(25));
  + our_time.sleep_for(std::chrono::milliseconds(125));
  for (int i = 0; i < tasks_num; ++i) {
  EXPECT_EQ(tasks[i], i);
  