xorg-gtest tests don't handle the case of a crashing compiz correctly

At the moment, our integration tests just unilaterally terminate the compiz process once it has finished examining X events. It doesn't allow it to gracefully shut down, and then check the exit status. That means that compiz can crash in the tests and the test will be marked as "passed" if all of the postconditions were fufilled, which seems counterintuitive.