Use Thread.pass in loop{} to check interrupts more often

* The spec now runs in ~5ms vs ~100ms before.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63256 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/thread/raise_spec.rb

@@ -151,7 +151,7 @@ describe "Thread#raise on a running thread" do
         1/0
       rescue ZeroDivisionError
         raised = true
-        loop { }
+        loop { Thread.pass }
       end
     end
     begin