UDN: Add missing external -> services open flow related tess.

[dceara]

What would you like to see added/updated/deleted in our unit tests?

For UDNs (user defined networks) add tests to validate that the corresponding openflow rules are properly rendered (added/deleted/updated) in br-ex (breth0). For each node port service a bunch of flows are added to that bridge, e.g.: https://github.com/ovn-org/ovn-kubernetes/blob/834fd00de6ff09304b6bc5f274236725cfeb79ca/go-controller/pkg/node/gateway_shared_intf.go#L308-L309

Why is this needed?

Improved unit testing of the UDN feature.