Add infrastructure to run tests under GTestDBus

Add infrastructure to run tests under GTestDBus

For now, all library TestCase subclasses except the one for key-files
override this back to "do nothing" and rely on being run under
with-session-bus.sh, because I haven't checked whether they survive
having the D-Bus session bus disconnected from under them.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=690830
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>
[added comments about the undesirable dbus-1 dependency -smcv]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>