There is a session bus setup by systemd for the normal user, which is
the one all tests should use. Use that by simply running the test as
user user.

Fixes: https://phabricator.apertis.org/T1906



Reviewed-by: Luis Araujo <luis.araujo@collabora.co.uk>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3662