The name is not as universal as we want, still, hence let's use our own
user we create with sysusers.d/. That should yield same behaviour
everywhere (and also test sysusers a bit as side effect).
To support ProtectHome=y in a user namespace (which mounts the inaccessible
nodes), the nodes need to be accessible by the user. Create these paths and
devices in the user runtime directory so they can be used later if needed.
The test exercises that PrivateTmp=yes and ProtectHome={read-only,tmpfs}
directives work as expected when PrivateUsers=yes in a user manager.
Some code is also added to test-functions to help set up test cases that
exercise the user manager.
