Add e2e tests for v(N - M) -> vNalpha

[htuch]

We should be able to reuse almost all of the xDS client code and tests for v3, but we will need to parameterize the code on API version.

[htuch]

After discussing with maintainers, the plan is to have all tests operate on the latest vNalpha. We will have some limited integration tests created that exercise the v(N-M) -> vNalpha and configuration ingestion e2e, this issue will track these instead (changing title to reflect).

[htuch]

Fixed in #9526