evaluate assertions at appropriate time

Evaluate assertions at appropriate time in XMLHttpRequest/abort-event-order.html, to compliment
10010b1's corrected expected results

Closes #958
assert readyState updated after abort() in nextTick

Per the spec, the readyState is updated after the events are dispatched. Since the
assertion occurs during the final loadend event handler, there must be a pause before
the readyState can be tested, as browsers behaving according to the spec will not
have updated the readyState yet.
Merge pull request #959 from caitp/issue-958

evaluate assertions at appropriate time