Add a test that opens and closes browser windows (as opposed to tabs)

[staktrace]

We had a leak recently (bug 1264161) where every time a browser window was opened and closed it would leak a bunch of stuff. Surprisingly (to me) none of our automation tests for this, since leak checking is done at shutdown which was too late for this particular scenario. AWSY however would have caught this sort of a leak if it opened and closed windows in addition to tabs. Something to consider adding, and I imagine it shouldn't be too hard. It could be as simple as an extra phase at the end of the test (after "After TP5, tabs closed [+30s, forced GC]") where we cycle through the TP5 set opening and closing each page in a new window, then do a force GC and grab another snapshot. I strongly suspect that such a test would have showed the leak.

[staktrace]

https://bugzilla.mozilla.org/show_bug.cgi?id=1277358 is another bug that we can probably test for in AWSY.