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

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

By using the auto-generated NVRAM path in test data files, we won't see
bugs where a user specified path gets accidentally overwritten by a
post-parse callback, or VM startup. For example, this caused us to miss
the bug fixed by:

  commit 24adb6c7a6cbd8ce5f9de67e6af5d89e0e57c270
  Author: Michal Prívozník <mprivozn@redhat.com>
  Date:   Wed Feb 23 08:50:44 2022 +0100

    qemu: Don't regenerate NVRAM path if parsed from domain XML

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>