tests: Make tests fail if they try to connect to the real session bus

tests: Make tests fail if they try to connect to the real session bus

It is too easy for a developer working in an environment that has a
session bus to write tests that pass locally, but fail in minimal
environments. This is also risky because the tests might do
destructive things on the developer's real session bus. We can avoid
connecting to the session bus by consistently removing its address
from the environment, and replacing it with something that will
always fail.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101698