tests: improve assertion when forbidden events occur
tests: improve assertion when forbidden events occur

This makes the backtraces prettier; it doesn't have any functional
effect.

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