kunit/fortify: Fix memcmp() test to be amplitude agnostic

When memcmp() returns a non-zero value, only the signed bit has any
meaning. The actual value may differ between implementations.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://github.com/ClangBuiltLinux/linux/issues/2025


Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240518184020.work.604-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>