Update initial DNS seedlist discovery tests to support dedicated load balancer port

DRIVERS-2224 : The driver updates required in DRIVERS-1983 break the initial-dns-seedlist-discovery load-balanced tests because those tests still point the driver directly at the mongos instances, requiring that the driver still mocks the service ID. See this comment for more details about the test failures in the Go driver. Update those tests and the associated DNS records to point the driver at the load balancer started by run-load-balancer.sh, allowing drivers to stop mocking service ID. Here's the ticket for updating the DNS records: https://jira.mongodb.org/browse/BUILD-14668 Here's a PR for the test changes: https://github.com/mongodb/specifications/pull/1148 Here's the draft PR testing the spec test changes in the Go driver: https://github.com/mongodb/mongo-go-driver/pull/861