selftests: txtimestamp: allow printing latencies in nsec.

Txtimestamp reports latencies in uses resolution, while nsec is needed
in cases such as measuring latencies on localhost.

Add the following new flag:
-N: print timestamps and durations in nsec (instead of usec)

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