Add tests to pkg/agent/loadbalancer

[brandond]

The loadbalancer package is somewhat complicated, and is critical to inter-node communication. It maintains a fair bit of state and has many code paths that hold locks.

We need to add tests to this package to ensure that the loadbalancer behaves as intended at all times, and that changes do not trigger hard to diagnose failures, lock-related or otherwise. In particular, we should test concurrent failover, health checking, and server addition/removal, as these are paths that will hold interleaving read and write locks.