test/py: efi_secboot: adjust secure boot tests to code changes

The previous patch is changing U-Boot's behavior wrt certificate based
binary authentication.  Specifically an image who's digest of a
certificate is found in dbx is now rejected.  Fix the test accordingly
and add another one testing signatures in reverse order

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>