efi: test/py: authenticate fit capsules

Add support for the authentication of UEFI capsules containing FIT images.

The authentication code is moved out of the function handling raw images
into a new function efi_firmware_capsule_authenticate(). The special case
for the FMP header coming from edk2 tools is preserved. There is no
functional change for capsules containing raw images.

The python test for signed capsules with raw images is renamed with no
functional change and a new test is added for signed capsules containing
FIT images.

This can be tested with sandbox64_defconfig or sandbox_flattree_defconfig,
plus CONFIG_EFI_CAPSULE_AUTHENTICATE=y.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>