support/testing: add polkit tests

support/testing: add polkit tests

This test script tests polkit with and without systemd.

The Systemd test does the following:
  - The brtest user attempts to restart the systemd-timesyncd service and is
    denied.

  - A systemd-timesyncd-restart.rules file provided by polkit-rules-test
    is copied from /root/ to /etc/polkit-1/rules.d

  - The brtest user attempts to restart the systemd-timesyncd service and should
    now succeed.

The initd test does the following:
- The brtest user attempts to run the test application "hello-polkit" with the
  command "pkexec hello-polkit" and is denied.

- A hello-polkit.rules file provided by polkit-rules-test is copied from /root/
  to /etc/polkit-1/rules.d

- The brtest user attempts to re-run the test hello-polkit binary with
  "pkexec hello-polkit" and succeeds.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>