test: Add a sysupdate test for files which are a prefix match of each other

test: Add a sysupdate test for files which are a prefix match of each other

This tests whether the pattern matching code checks it’s matched the
whole string and not just a prefix (see commit 4ffb60319b).

In particular, this tests a setup which KDE currently use in their
sysupdate images, where two regular file transfers are done, one of a
`foo.erofs` file, and the other `foo.erofs.caibx`. As one is a prefix of
the other, they were hitting this bug.

See:
 - https://files.kde.org/kde-linux/sysupdate/v2/
 - https://github.com/KDE/kde-linux/tree/master/mkosi.extra/usr/lib/sysupdate.d

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Fixes: https://github.com/systemd/systemd/issues/38605
Fixes: https://github.com/systemd/systemd/issues/41288