CLI --watch accomulates error handlers

[Krinkle]

Tell us about your runtime:

• QUnit version: 2.18.2
• Environment: Node

What actually happened?

What did you expect to happen?

Ultimately, to work correctly and not cause the memory leak warning.

Concretely, we probably either need to keep only one handler for the Node environment and use whichever the last initialised QUnit instance was from the same shared lexical scope (or global), or to properly remove the lister after each run, and re-add it on restart.