Add Imp test for checking that vote delegations are removed when unregistering and then reregistering a DRep

[carlostome]

In a transaction with two certificates that:

1. unregisters a DRep, and
2. reregisters the same DRep
   the implementation removes the vote delegations and thus after the tx is applied to the ledger state the DRep shouldn't have vote delegations.

A possible variation of this test could add new delegations after the DRep has been reregistered, which should exist in the ledger state after the tx has been applied.

Note: The spec differs in this matter wrt the implementation, therefore such test should fail in conformance.