If a process that we start via systemd exits really quickly,
it can go to inactive state before CbyServiceManager is able to find
out the process ID of the main process. CbyServiceManager's API
currently guarantees the process ID is available, for the benefit of
the Shapwick integration in canterbury-full; so if the main process
exits too soon, CbyServiceManager has no choice but to consider the
activation to have failed.

As far as I can tell, this could have happened any time, but developing
a test for agents as part of T2776 and running that test first seems
to be enough to nudge the timing from "rarely happens" to "nearly
always happens". Hopefully this change will make the launch-preferences
test more stable.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Frédéric Dalleau <frederic.dalleau@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D6987