Adjust policykit policy

[slakkala]

Description

Allow set-timezone and set-locale actions for all users in "users" group.
Also add simple tests for the set-timezone and set-locale commands.

Checklist

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Test procedure added to nixos/tests
• Author has run nix flake check --accept-flake-config and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing