selftests: mlxsw: Add a test case for L3 VNI

Previous patch added the ability to offload a VXLAN tunnel used for L3
VNI when it is present in the VLAN-aware bridge before the corresponding
VLAN interface is configured. This patch adds a test case to verify
that.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>