selftests: forwarding: lib: Add ping6_, ping_test_fails()

Add two helpers to run a ping test that succeeds when the pings themselves
fail.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>