test: perform /usr/share/ sysext unmount lazily

test: perform /usr/share/ sysext unmount lazily

/usr/share/ is a directory commonly accessed by various tools, hence we
really should make sure we umount it lazily (MNT_DETACH), since
otherwise there's a good chance that the umount might simply fail.

I noticed this there:

https://github.com/systemd/systemd/actions/runs/21321690161/job/61372465563?pr=40446
(cherry picked from commit 2e38c56c73b06e4e031535ad2dccbb6d065fbadd)
(cherry picked from commit b6b0c83b8d1602925ab918e4ae1c4245a2a4c128)
(cherry picked from commit a88ff44c4405db05fec6f2e0364ff9952007599d)