selftests: forwarding: mirror_lib: 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. Those
are mirrored, and over a certain threshold the test case run is
considered a failure, because too much traffic is observed.

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>