test/py: efi_capsule: test for raw image capsule

The test can run on sandbox build and it attempts to execute a firmware
update via a capsule-on-disk, using a raw image capsule,
CONFIG_EFI_CAPSULE_RAW.

To run this test successfully, you need configure U-Boot specifically;
See test_capsule_firmware.py for requirements, and hence it won't run
on Travis CI, at least, for now.

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