Fix leaking global 'uncaughtException' handler

[juergba]

Description

const mocha = require("mocha");
for (let i = 0; i < 15; i++) {
    const r = new mocha.Runner(new mocha.Suite(""+i, undefined));
    r.run();
}

When using Mocha programmatically with multiple runners, a MaxListenersExceededWarning is emitted by Node. For every runner instance an uncaughtException handler is registered, but never removed.
This is a regression.

Description of the Change

Remove eventual uncaughtException handler before starting a new runner instance.

Applicable issues

closes #4144

[coveralls]

Coverage increased (+0.04%) to 92.85% when pulling a4cfa86 on juergba/event-leak into 7d78f20 on master.

[juergba]

@weswigham could you run a test with this batch in your environment, please?

[weswigham]

Yep, I can confirm that this branch fixes the warnings in our environment.

[weswigham]

Is there going to be a minor bump and publish with this patch at some point?

[juergba]

We have two additional PR's regarding uncaught exceptions in process. I would like to have them merged first.