selftests: forwarding: Add some missing configuration symbols

For the forwarding selftests to work, we need network namespaces when
using veth/vrf otherwise ping/ping6 commands like these:

ip vrf exec vveth0 /bin/ping 192.0.2.2 -c 10 -i 0.1 -w 5

will fail because network namespaces may not be enabled.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>