test: wait for unit generated from /proc/self/mountinfo to be unloaded

Fixes systemd/systemd#32680 (comment).
===
May 21 02:45:08 TEST-74-AUX-UTILS.sh[2475]: + mountpoint /tmp/tmp.eaRV7lSbX2/mnt
May 21 02:45:08 TEST-74-AUX-UTILS.sh[2476]: /tmp/tmp.eaRV7lSbX2/mnt is not a mountpoint
May 21 02:45:08 TEST-74-AUX-UTILS.sh[2449]: + systemd-mount /dev/loop0 /tmp/tmp.eaRV7lSbX2/mnt
May 21 02:45:08 systemd-mount[2477]: Failed to start transient mount unit: Unit tmp-tmp.eaRV7lSbX2-mnt.mount was already loaded or has a fragment file.
===

(cherry picked from commit 4a8ca3c)
(cherry picked from commit 1a6a2d8)
(cherry picked from commit 861affe)

Author: yuwata

test/units/testsuite-74.mount.sh

@@ -39,6 +39,9 @@ mount "$LOOP" "$WORK_DIR/mnt"
 touch "$WORK_DIR/mnt/foo.bar"
 umount "$LOOP"
 (! mountpoint "$WORK_DIR/mnt")
+# Wait for the mount unit to be unloaded. Otherwise, creation of the transient unit below may fail.
+MOUNT_UNIT=$(systemd-escape --path --suffix=mount "$WORK_DIR/mnt")
+timeout 60 bash -c "while [[ -n \$(systemctl list-units --all --no-legend $MOUNT_UNIT) ]]; do sleep 1; done"
 
 # Mount with both source and destination set
 systemd-mount "$LOOP" "$WORK_DIR/mnt"