test/py: efi_secboot: add a test for verifying with digest of signed image

Signature database (db or dbx) may have not only certificates that contain
a public key for RSA decryption, but also digests of signed images.

In this test case, if database has an image's digest (EFI_CERT_SHA256_GUID)
and if the value matches to a hash value calculated from image's binary,
authentication should pass in case of db, and fail in case of dbx.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

Use defined time stamps for sign-efi-sig-list.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>