rspec test cases for mutex 'sleep'

[rajcybage]

I saw that there was test cases for synchronize,lock,unlock but not for sleep
I have implemented some rspec test cases for the 'sleep' method of Mutex.

Also use expect rather than should
Thanks

[rajcybage]

cc @headius @enebo @BanzaiMan

[BanzaiMan]

This should go to our rubyspec repo (https://github.com/jruby/rubyspec). Please re-create it there.

[rajcybage]

There has already sleep_spec.rb for mutex but not present in our jruby specs.

I have one more question as we upgrade our rspec to 2.14 so can we use expect instead of should method http://betterspecs.org/#expect.
Please look into #966

This is just a suggestion

Thanks