Fix broken test.

Author: samuel-williams-shopify

test/async/service/managed/service.rb

@@ -77,6 +77,10 @@ def mock_instance.ready!
 				@ready_called = true
 			end
 
+			def mock_instance.healthy!
+				# health_checker calls this method periodically
+			end
+			
 			def mock_instance.status!(text)
 				# status! is called during startup but doesn't need to be tracked
 			end