test_thread.rb: ignore meaningless test

* test/ruby/test_thread.rb (test_blocking_mutex_unlocked_on_fork):
  ignore meaningless test on platforms where fork() is not supported.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43152 b2dd03c8-39d4-4d8f-98ff-823fe69b080e