rabbit_ct_helpers: Don't stop testsuite monitor if it's not running

This could happen if the setup run steps failed before we start the
testsuite monitor. In this case, `?config()` was returning `undefined`
and we tried to use it as a PID.

deps/rabbitmq_ct_helpers/src/rabbit_ct_helpers.erl

@@ -461,7 +461,10 @@ load_elixir(Config) ->
     Config.
 
 stop_long_running_testsuite_monitor(Config) ->
-    ?config(long_running_testsuite_monitor, Config) ! stop,
+    case get_config(Config, long_running_testsuite_monitor) of
+        undefined -> ok;
+        Pid       -> Pid ! stop
+    end,
     Config.
 
 long_running_testsuite_monitor(TimerRef, Testcases) ->