NextVe: Implements IMigrator Interface

[rareweasel]

The VotingEscrow.vy contract uses a Migrator interface internally, but the current test implementation doesn't use an interface.

My suggestion is to create a IMigrator.sol interface and use it in the NextVe.sol test contract.

Note: It isn't a high priority. It is just to tracking and don't loss the tracking of the issue.

[pandadefi]

We will create it once we need it