All tests: switch to a calling convention that does not rely on leaking

All tests: switch to a calling convention that does not rely on leaking

The standard main() used in each test relied on the fact that
get_suite() kept a ref to the TestCase subclass instance, which would
be stored in the global variables of the GTest infrastructure,
resulting in the TestCase never being freed and its final_teardown()
never being executed.

That's undesirable if final_teardown() is going to do something
significant, like terminating a temporary session bus. As a first
step towards that, run the tests with the TestCase subclass instance
globally referenced (so that the version passed to GTest can eventually
be a weak reference), and explicitly run final_tear_down() so that
even if there is a ref leak (as there is now), it gets executed.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=695381
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>