test suite: root access control feature

This ticket tracks the progress of replacing certain steps of the manual test suite with automated tests in features/root_access_control.feature.

Tests that needs a review:

The following automated tests need a review from someone else than intrigeri, after which this ticket’s “Type of work” can be set to “Pass Test”. Then, once the tests pass for two of us, we can remove the corresponding tests from our manual test suite.

• Boot Tails without setting a password and check you can’t get PolicyKit administrative privileges (e.g. run a command with pkexec), neither with the amnesia password nor with the live one.
• Boot Tails with a password and check you can get PolicyKit administrative privileges.

Original created by @tails on 5431 (Redmine)