test/py: efi_capsule: repair image authentication test

Repair the python tests for authenticated EFI capsules, which can be run
with sandbox_defconfig plus CONFIG_EFI_CAPSULE_AUTHENTICATE=y.

- Account for the reset changes done by commit 3e6f8100 ("efi_loader:
  test/py: Reset system after capsule update on disk").
- Fix the capsule GUID typo introduced by commit 2e9c3c69 ("test:
  capsule: Modify the capsule tests to use GUID values for sandbox").

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