selftests: forwarding: fix race between packet receive and tc check

It is possible that tc stats get checked before the packet we check for
actually arrived into the interface and accounted for.
Fix it by checking for the expected result in a loop until
timeout is reached (by default 1 second).

Fixes: 07e5c751 ("selftests: forwarding: Introduce tc flower matching tests")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>