Removed thread and mutex_m dependencies

[hsbt]

Ruby core team have plan to extract mutex_m from stdlib at Ruby 3.4. After that, minitest needs to depend mutex_m gem.

I replaced mutex_m with simple Mutex usage. But I'm not familiar with thread programming. It may be wrong usage.

[eregon]

Suggested change

	def mutex # :nodoc:
	@mutex
	end
	def synchronize(&block) # :nodoc:
	@mutex.synchronize(&block)
	end

This would be cleaner interface-wise and avoid exposing the Mutex.
Also it would avoid needing changes in lib/minitest/parallel.rb

[hsbt]

👍 Thanks. It's better than my changes.

[zenspider]

DONE! Thank you. Sorry for the delay. I'll get this out in the next release shortly

[hsbt]

@zenspider Thank you!

[Earlopain]

@zenspider hope you don't mind the ping. When do you except to cut the next release? Ruby main has now removed mutex_m code and as such testing against ruby-head errors unless manually adding it to the gemfile/gemspec.

[zenspider]

Just released v5.21.0 with this fix.

[Earlopain]

Thank you, I appreciate that very much.

[hsbt]

Thanks always!