If spawn_dbus_daemon() can fail for TEST_USER_ME, then we'd have to
go through all the tests adding the ability to skip tests after
it fails, which is a fairly extensive change.
The tests have historically all run as whatever uid is supplied, and
if the tests are being run as root for some reason - perhaps in a CI
framework for an embedded platform that doesn't have non-root users,
or in an environment where you can be root or non-root but not both -
there is no particular reason to skip them.
In the process, make test_kill_pid() safer: do not try to terminate
more than one pid, or the NULL handle.
Also stop leaking the address_fd in spawn_dbus_daemon, a pre-existing
bug that was spotted by Philip Withnall during review.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=88810
Reviewed-by: Philip Withnall
