servicetest: require signature when emitting D-Bus messages
servicetest: require signature when emitting D-Bus messages

Otherwise, it's very easy to get it wrong, which means that dbus-glib
will silently ignore the message. Thanks, dbus-glib.

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control