tests: limit backends via the environment a bit more directly

tests: limit backends via the environment a bit more directly

This makes it more obvious what's enabled and what isn't. We want to
make sure that only the intended backends are enabled, because those
are the ones for which we've done enough environmental setup
(D-Bus, etc.) to make sure they don't "leak out" into the user's
real data.

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>