selftests: mlxsw: vxlan: Add test for error path

Test that when two VXLAN tunnels with conflicting configurations (i.e.,
different TTL) are enslaved to the same VLAN-aware bridge, then the
enslavement of a port to the bridge is denied.

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