Use Thread.pass until thread.stop? to wait for thread to block

[Bug #19415]

It should be more reliable

spec/ruby/fixtures/code/concurrent_require_fixture.rb

@@ -1,4 +1,4 @@
 object = ScratchPad.recorded
 thread = Thread.new { object.require(__FILE__) }
-thread.wakeup unless thread.stop?
+Thread.pass until thread.stop?
 ScratchPad.record(thread)