selftests: txtimestamp: add flag for timestamp validation tolerance.

The txtimestamp selftest sets a fixed 500us tolerance. This value was
arrived at experimentally. Some platforms have higher variances. Make
this adjustable by adding the following flag:

-t N: tolerance (usec) for timestamp validation.

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