Remove hardcoding of the session environment

Our test should be using the session environment as setup by the system
(in lava specifically by run-in-systemd). So remove the hardcoding of
DISPLAY (deprecated on wayland targets anyway), XDG_RUNTIME_DIR (setup
by the environment in all cases) and DBUS_SESSION_BUS_ADDRESS (should be
inferred by XDG_RUNTIME_DIR by all supported dbus libraries)

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/D3664