Fix test of GVL instrumentation on Ractor sleeping

It seems that the Ractor sleep GVL event arrives very slightly after the
value becomes available and other threads wake (which makes sense) so we
need a little additional time to ensure we end up in a consisteny state.

test/-ext-/thread/test_instrumentation_api.rb

@@ -172,6 +172,7 @@ class TestThreadInstrumentation < Test::Unit::TestCase
           sleep 0.1
           Thread.current
         }.take
+        sleep 0.1
       end
 
       timeline = timeline_for(thread, full_timeline)