selftests: mlxsw: Update ping limits

The current ping intervals are too short for running mirroring tests in
simulator. This leads to ping sending a follow-up ping before the reply
arrives, thus sending more than the requested 10 ICMP requests. This
traffic is seen at the counters, and causes spurious failures.

Bump interval and timeout numbers 5x in mirroring tests to address the
spurious failures.

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