test/rake: avoid leaking threads

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

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46207 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rake/test_rake_task.rb

@@ -13,6 +13,7 @@ class TestRakeTask < Rake::TestCase
 
   def teardown
     Rake::TaskManager.record_task_metadata = false
+    Rake.application.thread_pool.join
 
     super
   end