uncaughtException: fix "--allow-uncaught" with "this.skip()"

[juergba]

Description

describe('test suite', () => {
  it('test1', function () { });
  it('test2', function () {
    this.skip();
  });
  it('test3', function () { });
  it('test4', function () {
    this.skip();
  });
});

mocha --allow-uncaught test.js silently aborts execution after the first this.skip():

test suite
    √ test1

Our docu states:

[...] It's also important to note that calling this.skip() will effectively abort the test.

We achieve this abortion of the test/hook execution by throwing a Pending exception. In async scenarios this results in an intended uncaughtException which has to be swallowed somehow. Obviously --allow-uncaught does not distinguish correctly between "bad" uncaughtException (user code) and "good" uncaughtException (this.skip()).

Description of the Change

• independently of --allow-uncaught, we never allow the Pending exceptions to propagate.
• directly in Runnable#skip() we set the test to pending and immediately catch/swallow the Pending exception (sync scenario).
• for async this.skip() we register an addional uncaughtException handler for exceptions bubbling up after EVENT_RUN_END.
• the only purpose of the Pending is to abort the test execution, we don't use it for anything else.

[coveralls]

Coverage increased (+0.05%) to 92.781% when pulling f8f3542 on juergba/uncaught-skip into 5f8df08 on master.

[juergba]

I will do some additional testing and then merge this PR within the next few days.

[weswigham]

This is adding a new, unique uncaughtException handler every time the end event of a runner is called - this means for every finished runner, a new handler gets added (which is never removed). This, in turn, causes a Possible EventEmitter memory leak detected. 11 uncaughtException listeners added to [process]. Use emitter.setMaxListeners() to increase limit warning to be fired by node.

:(