test: fix test-mount-util when handling duplicate mounts on the same location

test: fix test-mount-util when handling duplicate mounts on the same location

The test was written so far under the assumption that if two mounts are
placed onto the same location the "upper" mount is listed later in
/proc/self/mountinfo. This appears not to be guaranteed however, as
running the tests in a normal nspawn shows.

This patch fixes that: it reverses the hashmap of mounts we build:
instead of keying by path, we key by mnt_id, and if we notice that
path_get_mnt_id() doesn't match what a line in /proc/self/mountinfo
says, we use the returned ID to check if maybe another line agrees.

Fixes: #7431