multiTestContext runs multiple Stores without Servers, and to do so it has to mock out the transport layer. Originally this was done because it was not feasible to use the regular Gossip and DistSender in this environment, but now we've moved closer to the standard environment and the last piece missing is the Server. We should go the rest of the way and move these tests to TestCluster, adding features to TestCluster as needed.

The immediate motivation for this change is deadlocks that result from multiTestContext using one RWMutex for too many things: #8170 and #7678.