public void suspend(Collection<String> threadNames) {
    if (threadNames == null) {
      suspend = true;
    } else {
      timeSuspendedThreads.addAll(threadNames);
    }
  }
Suspends the passage of time when {@link #sleep(long)} (or related functions) are called from the given
{@code threadNames}. Those functions still continue to return immediately.
If {@code threadNames} is {@code null}, suspends passage of time for all threads
@param threadNames the names of the threads for which the passage of time on sleep() should be suspended. If
                   {@code null}, it is suspended for all threads
ambry-test-utils/src/main/java/com/github/ambry/utils/MockTime.java