test/rake: avoid leaking threads

* test/rake/test_rake_multi_task.rb (teardown): clean up thread
  pool to avoid leaking threads.

* test/rake/test_rake_task_with_arguments.rb (teardown): ditto.

* test/rake/test_rake_thread_pool.rb: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46206 b2dd03c8-39d4-4d8f-98ff-823fe69b080e