selftests: forwarding: Make ping timeout configurable

The current timeout (2 seconds) proved to be too low for some (emulated)
systems where we run the tests.

Make the timeout configurable and default to 5 seconds.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>