test: rework dmsetup test to wait for device to disappear

test: rework dmsetup test to wait for device to disappear

There might be a delay between an umount and a refcounted device
to disappear, so the test can be flaky:

[   36.107128] TEST-50-DISSECT.sh[1662]: ++ dmsetup ls
[   36.108314] TEST-50-DISSECT.sh[1663]: ++ grep loop
[   36.109283] TEST-50-DISSECT.sh[1664]: ++ grep -c verity
[   36.110284] TEST-50-DISSECT.sh[1360]: + test 1 -eq 1
[   36.111555] TEST-50-DISSECT.sh[1360]: + umount -R /tmp/TEST-50-IMAGES.hxm/mount
[   36.112237] TEST-50-DISSECT.sh[1668]: ++ dmsetup ls
[   36.113039] TEST-50-DISSECT.sh[1669]: ++ grep loop
[   36.113833] TEST-50-DISSECT.sh[1670]: ++ grep -c verity
[   36.114517] TEST-50-DISSECT.sh[1360]: + test 0 -eq 1
[   36.116734] TEST-50-DISSECT.sh[1000]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/TEST-50-DISSECT.dissect.sh failed'

https://github.com/systemd/systemd/actions/runs/19062162467/job/54444112653?pr=39540#logs

Switch to searching for the dm entry and check for it specifically,
and wait for it to disappear before checking that it is no longer
in the dm table.

Follow-up for 10fc43e504da5962fa5f04341ae8ba92a9981be9