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: https://github.com/nodejs/node/pull/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>