Validate state migration for ddcloud_server (v3 to v4)

As mentioned in #102, it looks like the state migration for `ddcloud_server` [from v3 to v4](https://github.com/DimensionDataResearch/dd-cloud-compute-terraform/blob/1b26fb22685a573aef7dec14da418e28c7c50f43/ddcloud/resource_server_migrate.go#L221) has never been run.

Create acceptance tests to verify that the state data is correctly migrated.