Add subclasses for each category of TestCase

Add subclasses for each category of TestCase

Move small amounts of initialization (just the low-hanging fruit)
into each subclass. Folks.TestCase is now reserved for tests that
don't need any backends or additional services at all.

In the process, make TpTestsBackend.tear_down() idempotent, so it's OK
if we call it repeatedly.

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>