xds: refactor XdsClient test to cover protocol version v2 and v3 (#7577)

This change refactors client side XdsClient's unit test. The main testing logic (test cases) will being the abstract class while the extended classes will be providing xDS version-specific services and messages. With this approach, we do not suffer from maintaining two copies of test logics in order to cover both v2 and v3 xDS protocols. So every time making changes to XdsClient's own logic, we only need to modify the corresponding test logic in the abstract class. Also, this approach could be sustainable for future xDS protocol version upgrades without necessity to re-implement test logics.