This fixed the build failure of this test.
Python module crypt has been removed since python 3.13.
This PR uses `mkpasswd` to generate the same password for testing.
I have changed the name of the assert_password_match to indicate that
this only woks for sha512crypt algorithm.
This commit adds two new tests to show that the ordering of password
overrides documentation in nixos/modules/config/user-groups.nix is
correct. The override behavior differs depending on whether a system
has systemd-sysusers enabled, so there are two tests.