tests: Add tests for SEV firmware selection

tests: Add tests for SEV firmware selection

One of the new test cases demonstrates how firmware
autoselection doesn't currently work correctly for domains
using SEV-SNP: the descriptor for a suitable firmware exists,
and yet it doesn't get picked up.

Another test cases shows that, while firmware autoselection
succeeds for non-SNP SEV domains, the results are not the
expected ones: the generic (stateful) edk2 build is used
instead of the SEV-specific (stateless) one. This will be
addressed in a future patch by updating the descriptors.

Finally, one test case provides coverage for the uncommon
scenario of stateful firmware being explicitly requested by
the user.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>