* scheduler.unblock was already already called before but with no corresponding scheduler.block
* add test that Queue#pop makes the scheduler wait until it gets an element.
* Enables Mutex to be used as synchronization between multiple Fibers
of the same Thread.
* With a Fiber scheduler we can yield to another Fiber on contended
Mutex#lock instead of blocking the entire thread.
* This also makes the behavior of Mutex consistent across CRuby, JRuby and TruffleRuby.
* [Feature #16792]
