test/dbus-daemon: Exercise the Peer interface

test/dbus-daemon: Exercise the Peer interface

We have to skip the GetMachineId() part during build-time testing
if it wouldn't work - there is no guarantee that dbus has ever been
installed on the build system. However, we can insist on it during
installed-tests, if we make sure to complete the installation for the
Travis-CI build by running dbus-uuidgen.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101257