trash test: Don't rely on being able to determine mount points

If we can't find the mount point for target or tmp (as currently
happens on Launchpad autobuilders, and perhaps relatedly, on a
development system that uses btrfs), that's probably not great but is
not really the point of this test.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 13282768)
9 jobs for trash-mount-points in 13 minutes and 18 seconds (queued for 2 seconds)