Enhance stub balancer API to be able to wrap a real balancer underneath

[easwars]

We have a bunch of repeated code in our tests where we have a test balancer implementation which wraps a real balancer, because some behavior like creation of subConns once addresses are pushed to the balancer are required in the test. The test balancer would also be doing some other things like pushing some events on the channel.

We could get rid of all this repeated/duplicate code if we can get the stub balancer to support a real balancer underneath, along with its existing functionality.

https://github.com/grpc/grpc-go/blob/master/internal/balancer/stub/stub.go