nixos/tests/chromium: Run tests as normal user

The tests have failed because Chromium has started up displaying the
following error message in a dialog window:

  Chromium can not be run as root.

  Please start Chromium as a normal user. If you need to run as root for
  development, rerun with the --no-sandbox flag.

So let's run as user "alice" and pass all commands using the small
helper function "ru" (to keep it short, it's for "Run as User").

Tested it by running the "stable" test on x86_64-linux.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: @globin