tests: Skip system bus test if we are root but messagebus does not exist

Some CI systems do the build as root in a disposable container, and
run tests without ever having installed dbus. This means we can't
expect to be able to drop privileges from root to the DBUS_USER (usually
named messagebus or dbus) unless we have checked that the
DBUS_USER exists.

Signed-off-by: Simon McVittie <smcv@collabora.com>