selftests: vrf: Add test for SNAT over VRF

Commit 09e856d5 ("vrf: Reset skb conntrack connection on VRF rcv")
fixes the "reverse-DNAT" of an SNAT-ed packet over a VRF.

This patch adds a test for this scenario.

Signed-off-by: Lahav Schlesinger <lschlesinger@drivenets.com>
Signed-off-by: David S. Miller <davem@davemloft.net>