selftests/seccomp: Refactor to use fixture variants

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

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Will Deacon <will@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>