selftests: forwarding: lib: extract ping and ping6 so they can be reused

Extract ping and ping6 command execution so the return value can be
checked by the caller, this is needed for port isolation tests that are
intended to fail.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>