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

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

Fixes https://github.com/systemd/systemd/issues/32680#issuecomment-2120974685.
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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.
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