Test: Run server config auth checks with actual passwords

[roosterfish]

Follow up on an open discussion in https://github.com/canonical/lxd/pull/13373#discussion_r1589342963.

Instead of using an arbitrary user.foo config key, the test suite is modified to perform the authentication checks when trying to read and modify config keys that contain actual passwords. Currently those are core.trust_password and loki.auth.password.