Bug 689291 test-tabs.test window focus changes active tab | Test output exceeded timeout (60s)

* Using a safer method to close the window
* adding some more test asserts so that we have more information if there continue to be failures
Merge pull request #1169 from erikvold/689291

Bug 689291 test-tabs.test window focus changes active tab | Test output exceeded timeout (60s). r=Mossop