Ignore asynchronous errors after global failure

[jugglinmike]

Runner#uncaught labels the currently-executing Runnable instance as
"failed" in response to uncaught exceptions. If the the Runnable
instance later signals failure through Mocha's asynchronous interface,
the Runner counts this as a distinct test failure. Consequently,
reporters generate inaccurate reports (and in some cases, this triggers
uncaught exceptions causing the process itself to crash).

When a runnabe completes, explicitly check that it has not already been
labeled as failing before reporting any additional errors.

[danielstjules]

I'd noticed this before as well. And your PR looks good for the test case you mentioned. To highlight, for project maintainers. Given the following test, which only has one spec:

it('fails exactly once', function(done) {
  setTimeout(function() {
    setTimeout(function() {
      done(new Error('test error'));
    }, 0);
    throw new Error('global error');
  }, 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)

But while the PR does handle the case highlighted above, it doesn't fix all. Take, for example:

it('fails exactly once', function(done) {
  setTimeout(function() {
    done(new Error('test error'));
  }, 0);

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

Even with the proposed changes, you still get:

$ ./bin/mocha pr1540.js

  ․․

  0 passing (6ms)
  2 failing

  1)  fails exactly once:
     Uncaught Error: global error
      at null._onTimeout (/Users/danielstjules/git/mocha/pr1540.js:7:11)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)

  2)  fails exactly once:
     Uncaught Error: global error
      at null._onTimeout (/Users/danielstjules/git/mocha/pr1540.js:7:11)
      at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)

[jugglinmike]

Right on! Thanks for the assist, @danielstjules

[danielstjules]

No problem, teamwork ✋ haha