selftests: lib.sh: Add PING_COUNT to allow sending configurable amount of packets

Currently `ping_do()` and `ping6_do()` send 10 packets.

There are cases that it is not possible to catch only the interesting
packets using tc rule, so then, it is possible to send many packets and
verify that at least this amount of packets hit the rule.

Add `PING_COUNT` variable, which is set to 10 by default, to allow tests
sending more than 10 packets using the existing ping API.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>