If sh is not bash, some builtin command behave slightly differently.
E.g. if sh is provided by busybox, its builtin test command does not check if
the path is a mount point or not, and 'test -w' only checks the access mode of
the inode. So, even if a readonly filesystem is mounted on a directory,
the test command may succeed.
To avoid such confusion, let's unconditionally use bash.
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