xds/cdsbalancer: change tests to use xds resolver (#8579)

Change the tests in xds cds balancer to use xds resolver instead of
manual resolver

This change is being done as part of gRFC [A74 : xDS Config
tears](https://github.com/grpc/proposal/blob/master/A74-xds-config-tears.md).
This is to make sure the tests pass after the change too.

RELEASE NOTES: None

Author: eshitachandwani