kselftest: factor out list manipulation to a helper

Kees suggest to factor out the list append code to a macro,
since following commits need it, which leads to code duplication.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>