Ignore asynchronous failures after spec resolution

[danielstjules]

This improves on #1540 From that thread:

Given the following test, which only has one spec:

it('fails exactly once when a global error is thrown first', function(done) {
  setTimeout(function() {
    throw new Error('global error');

    setTimeout(function() {
      done(new Error('test error'));
    }, 0);
  }, 0);
});

You get two two failures:

$ ./bin/mocha pr1540.js

  ․․

  0 passing (6ms)
  2 failing

  1)  fails exactly once:
     Error: test error
      at null._onTimeout (/Users/danielstjules/git/mocha/pr1540.js:10:12)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
      at timers.js:117:18
      at process._tickCallback (node.js:415:13)
      at process._tickFromSpinner (node.js:390:15)

  2)  fails exactly once:
     Error: test error
      at null._onTimeout (/Users/danielstjules/git/mocha/pr1540.js:10:12)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
      at timers.js:117:18
      at process._tickCallback (node.js:415:13)
      at process._tickFromSpinner (node.js:390:15)

But after this PR, it correctly only shows one failure, and highlights the error thrown first ("global error"):

$ ./bin/mocha pr1540.js

  ․

  0 passing (6ms)
  1 failing

  1)  fails exactly once:
     Uncaught Error: global error
      at null._onTimeout (/Users/danielstjules/git/mocha/pr1540.js:12:11)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)

In addition to the above fix, this PR improves on #1540 by also correctly handling the following test case:

it('fails exactly once when a global error is thrown second', function(done) {
  setTimeout(function() {
    done(new Error('test error'));
  }, 0);

  setTimeout(function() {
    throw new Error('global error');
  }, 0);
});

It now also only reports a single failing spec, rather than two.

[travisjeffery]

Awesome! Thanks

[danielstjules]

No problem! :)