Mark continue_as_new as not implemented in testing context (#299)

Raise NotImplementedError instead of NoMethodError for clarify.

Author: taonic

lib/temporal/testing/local_workflow_context.rb

@@ -180,6 +180,10 @@ def fail(exception)
         raise exception
       end
 
+      def continue_as_new(*input, **args)
+        raise NotImplementedError, 'not yet available for testing'
+      end
+
       def wait_for_all(*futures)
         futures.each(&:wait)