fails:Mutex#sleep raises an ArgumentError if passed a negative duration
fails:Mutex#sleep pauses execution for approximately the duration requested
fails:Mutex#sleep unlocks the mutex while sleeping
fails:Mutex#sleep relocks the mutex when woken
fails:Mutex#sleep relocks the mutex when woken by an exception being raised
fails:Mutex#sleep returns the rounded number of seconds asleep
fails:Mutex#sleep when not locked by the current thread raises a ThreadError
fails:Mutex#sleep when not locked by the current thread raises an ArgumentError if passed a negative duration
