selftests/txtimestamp: Add more configurable parameters

Add a way to configure if poll() should wait forever for an event, the
number of packets that should be sent for each and if there should be
any delay between packets.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>