selftests: skbedit_priority: Test counters at the skbedit rule

Currently the test checks the observable effect of skbedit priority:
queueing of packets at the correct qdisc band. It therefore misses the fact
that the counters for offloaded rules are not updated. Add an extra check
for the counter.

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