This PR cleans up a bunch of tests which used to define custom balancers which wrapped an underlying balancer and added some functionality on top of it. Instead of defining the balancers outside of the test, this PR uses the support in the stub package to define them inline. This leads to shorter and more readable code.

The pattern followed is as follows:

• Whenever a custom test balancer needs to wraps another existing policy, we use the Init method to build the underlying policy and store it back in stub.BalancerData.Data, thereby getting access to the underlying balancer whenever needed from other methods.
• Whenever a custom test balancer needs to override any methods in the balancer.ClientConn passed to the underlying balancer, a new struct testCCWrapper which contains only the overridden functionality is defined. This keeps the changes local and easier to follow.

Also the clustermanager tests were creating a wrapped round_robin policy which could ignore balancer attributes in addresses. The round_robin LB policy was updated a while ago to use the addressMap. So, we don't need this anymore in these tests and can use the vanilla round_robin policy.

RELEASE NOTES: none