qemuxml2argvtest: Set timezone

qemuxml2argvtest: Set timezone

With my recent work on the test, both time() and localtime() are used.
While mocking the former one, we get predictable result for UTC. But
since the latter function uses timezone to get local time, the result of
localtime() is not so predictive. Therefore, we must set the TZ variable
at the beginning of the test. To be able to catch some things that work
just by a blind chance, I'm choosing a virtual timezone that (hopefully)
no libvirt developer resides in.