installcheck-twisted: test the just-installed MC if using a DESTDIR
installcheck-twisted: test the just-installed MC if using a DESTDIR

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=69542
Reviewed-by: Guillaume Desmottes <f16649c7afcd84e3350287183c4ed23cb1aea57c@collabora.co.uk>

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