test: verify listener leak is only emitted once

When a possible listener leak is detected, a warning is
emitted. This commit updates an existing test to verify that the
warning is only emitted once.

PR-URL: #12502
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Author: cjihrig

test/parallel/test-event-emitter-max-listeners-warning.js

@@ -19,4 +19,5 @@ process.on('warning', common.mustCall((warning) => {
 }));
 
 e.on('event-type', function() {});
-e.on('event-type', function() {});
+e.on('event-type', function() {});  // Trigger warning.
+e.on('event-type', function() {});  // Verify that warning is emitted only once.