Stop on first failed assertion

[jankuca]

I noticed that test cases are not stopped when an assertion fails. To be exact, they are stopped asynchronously. Here is an example:

test(t => {
  let arr = [ { name: 'abc' } ];
  t.is(arr.length, 2);
  t.is(arr[1].name, 'xyz');
  t.end();
});

The error I'd expect to see is "1 !== 2" while the actual error I get is "cannot read property 'name' of undefined" because execution is not stopped on the failed assertion (on the line above).

Is there a reason why Test#exit() is called asynchronously (by Test#_assert()) rather than synchronously (which is the case for errors thrown outside assertions)?

[jamestalmage]

Duplicate of #121

Fix PR: #259