tests: Don't set NVRAM path manually

tests: Don't set NVRAM path manually

This does the opposite of

  commit 392292cd99ee275f986e9e21f325a9fee8e8bbfe
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Feb 23 12:45:51 2022 +0000

    tests: don't use auto-generated NVRAM path in tests

in order to minimize input files.

We're going to add a test case specifically covering the use of
custom NVRAM paths with firmware autoselection in an upcoming
commit.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>