Prefix-To-Interface datastore is not cleaned up incase of Dual stack testcases

4 Dual-Stack test cases have a bug where, when Dual stack VM is deleted, one of the primary-adj entry remains stale in prefix-to-interface.

Analysis:

In oxygen dual-stack coming into picture, I see challenge in managing prefix-to-interface.

The reason being , prefix-to-interface is managed via VpnInterface Op listener like before(Boron).

But in Boron , prefix-to-interface always had only one primary adj stored , it was manageable through VpnInterfaceOp listener.

When adj count becomes 1 , we deleted the entire VpnInterface Op entry for the port. It used to trigger  remove() of VpnInterfaceOp listener.

In that ,  we removed prefix-to-interface and updated the vpn-to-dpn-list.

But challenge in Oxygen is , we have 2 primary adj  stored in VpnInterfaceOp DS. When VM is deleted , 2 FIB entries are removed.

In cleanupOpDataForFib() , when we remove first primary adj from VpnInterfaceOp DS , it triggers update in VpnInterfaceOp listener.

We ignore update of VpnInterfaceOp. When 2^nd^ / final primary adj is removed , we delete the VpnInterfaceOp entry.

This leads to removal of prefix-to-interface for 2^nd^ primary adj. First primary adj , always remains stale.