test/py: efi_secboot: add a test for multiple signatures

In this test case, an image is signed multiple times with different
keys. If any of signatures contained is not verified, the whole
authentication check should fail.

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

Provide a defined time stamp for dbx_hash1.auth.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>