sign_uefi_unittest.py: Fix long-line lint

Also replace "../" with `.parent`.

BRANCH=none
BUG=none
TEST=cros lint scripts/image_signing/sign_uefi*
TEST=make runtests

Change-Id: I06c5b3ec931dd7ae1b8c861e74704ab6db81be45
Signed-off-by: Nicholas Bishop <nicholasbishop@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/4799756
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed