We want to retain *some* of the full paths in order to test more code paths.
But the default should be to use the command name only. This makes the tests
less visually cluttered.
The read-only bit is flipped after setting up all the mounts, so that
bind mounts can be added. Remove the early config, and add a unit
test.
Fixes https://github.com/systemd/systemd/issues/30372