selftests: Make test_harness.h more generally available

The seccomp/test_harness.h file contains useful helpers to build tests.
Moving it to the selftest directory should benefit to other test
components.

Keep seccomp maintainers for this file.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Drewry <wad@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Shuah Khan <shuah@kernel.org>
Link: https://lkml.kernel.org/r/CAGXu5j+8CVz8vL51DRYXqOY=xc3zuKFf=PTENe88XYHzFYidUQ@mail.gmail.com


Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>