nixos/tests/installer: Fail on impure udev rules

With 9f2a61c5

 in place, let's actually use this in the installer tests to
make sure we won't shovel FHS paths down the throad of unstable channel
users.

I've tested this by running all of the installer tests for x86_64-linux
and they all succeeded.

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