[ruby/spec] Just test that sleep completes

spec/ruby/core/mutex/sleep_spec.rb

@@ -97,7 +97,7 @@ describe "Mutex#sleep" do
     m.lock
     times.each do |time|
       # just testing that sleep completes
-      m.sleep(time).should_not == nil
+      -> {m.sleep(time)}.should_not raise_error
     end
   end
 end