selftests: mlxsw: Add a test case for VLAN addition error flow

Add a test case for the issue fixed by previous commit. In case the
offloading of an unsupported VxLAN tunnel was triggered by adding the
mapped VLAN to a local port, then error should be returned to the user.

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