selftests/seccomp: Refactor to use fixture variants

Now that the selftest harness has variants, use them to eliminate a
bunch of copy/paste duplication.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>