selftests: expand txtimestamp with cmsg support

Commit 3dd17e63 ("sock: accept SO_TIMESTAMPING flags in socket
cmsg") added support for passing tx timestamping options per-call
in sendmsg.

Expand the txtimestamp test with support for this feature.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>