I added those tests because RubySpec doesn't cover neither [waitpid](https://github.com/rubyspec/rubyspec/blob/master/core/process/waitpid_spec.rb) nor [waitpid2](https://github.com/rubyspec/rubyspec/blob/master/core/process/waitpid2_spec.rb). However, given that some parts of RubySpec do call waitpid{,2} and depend on their correct behaviour I decided that we should have tests for them. I don't think they can be reliably tested without actualy forking. Mocking is mocking.