tests: Tweak descriptor for combined firmware

tests: Tweak descriptor for combined firmware

This kind of firmware build is not shipped in Fedora, where
most descriptors in our test suite come from, so we had to
make it up. It was based off the Secure Boot-enabled edk2
build, and the filename it points to is the same.

That has been fine so far since it's not actually being picked
up by any of the test cases, but that's going to change soon
and when it does we want to be able to avoid any confusion.

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