xds/clusterimpl: need better e2e tests

[easwars]

Current tests were not able to find the issue fixed by this PR: #6112

This is because they directly call UpdateClientConnState and pass it parsed service config. The clusterimpl LB policy like so many others in our tree of LB policies don't function in isolation. They work in concert with other LB policies. Our functional tests should be e2e style tests instead which rely on getting configuration from the management server, and have that config pass through the whole system before reaching the SUT.

As many of the existing tests should be re-written as e2e style tests.