tests: Fix firmware descriptor masking test

Right now we're checking that firmware descriptor masking works
as intended by creating an empty file matching 60-ovmf-sb.json
in name.

However, that firmware descriptors contains the details for a
perfectly valid and quite common situation: Secure Boot being
supported by the firmware build, but being effectively disabled
by the lack of certificates in the NVRAM template.

Unmask that firmware descriptor, and instead create a dummy one
that has higher priority than all other OVMF builds and points
to paths that are obviously incorrect, which should make it
easy to notice it getting accidentally unmasked in the future.

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