wait until the thread is stopped.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47667 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bootstraptest/test_thread.rb

@@ -85,7 +85,7 @@ assert_equal %q{ok}, %q{
       ans = :ok
     end
   }
-  Thread.pass
+  Thread.pass until t.stop?
   t.kill
   t.join
   ans