The kernel interface requires setting up read-only bind-mounts in
two steps, the bind first and then a read-only remount.
Fix nspawn-mount, and cover this case in the integration test.
Fixes#16484
The test would fail when run again from the same image. So let's
rename the stuff we create to be more unique, and remove it before
running the test. (Removing it after would be more elegant, but it's
hard to make sure that everything is removed when things fail halfway.
Cleanup *before* tests is much more rebust.)