test/py: efi_capsule: add image authentication test

test/py: efi_capsule: add image authentication test

Add a couple of test cases against capsule image authentication
for capsule-on-disk, where only a signed capsule file with the verified
signature will be applied to the system.

Due to the difficulty of embedding a public key (esl file) in U-Boot
binary during pytest setup time, all the keys/certificates are pre-created.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>