Add e2e style tests for error conditions in the LRS implementation

[easwars]

Existing LRS tests are being moved to xdsclient/tests package and are being made e2e style as part of this PR: #7694

We could add one more test to the above file to verify backoff behavior when the LRS stream implementation receives error from the management server.

Our load reporting API on the xDS client does not report errors returned by the server to the caller. So, the only thing we need to check in this test is to ensure that appropriate backoff happens when stream receives errors, and that backoff is reset when a message is received from the server.

We already have tests for these conditions for the ADS stream. See: https://github.com/grpc/grpc-go/blob/master/xds/internal/xdsclient/tests/ads_stream_backoff_test.go