selftests: forwarding: vxlan_bridge_1d: Add more ECN decap test cases

Test that all possible combinations of inner and outer ECN bits result
in the correct inner ECN marking according to RFC 6040 4.2.

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