test/py: efi_secboot: add a test for a forged signed image

In this test case, a image binary, helloworld.efi.signed, is willfully
modified to print a corrupted message while the signature itself is
unchanged.

This binary must be rejected under secure boot mode.

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

test/py/tests/test_efi_secboot/forge_image.sh

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+#!/bin/sh
+
+replace_exp="s/H\0e\0l\0l\0o\0/h\0E\0L\0L\0O\0/g"
+perl -p -e ${replace_exp} < $1 > $2