Rewrite responseText test for fake_xdomain_request#simulatetimeout

The previous test was testing for exceptions that it generated itself (through the `ontimeout` function in the setup), clouding the real value of the test: that responseText should be undefined after a timeout.

The rewritten test, only verifies that the `ontimeout` get's called and that responseText is undefined.
Stub out logError to reduce console spam in test runner
Merge pull request #520 from mroderick/silence-console

Silence console