tests: Add new test for round robin resolver

[jmhbnz]

The default grpc-go load balancing strategy is pick first which may not be expected, particularly for production scenarios. We should consider adding a test for round robin as an alternative.

Refer: etcd-io/website#633

[jmhbnz]

Useful context on grpc-go resolver within etcd: #15145

This pr may not be required based on the above issue, or alternatively be reverted at a later date once a new approach is merged.

[ptabor]

Thank you. Looks good. But I think that the number-of-calls assertion is crucial to verify it's really RR.

Probably most of the code could be shared with the previous test... just make the 'policy' options a parameter and post-run verification.

[tjungblu]

(non-binding) /lgtm

[jmhbnz]

Hey @serathius, @ahrtr - This pull request updates our existing test case testEtcdGrpcResolver to verify the resolver works as expected when using a round_robin strategy in addition to the default pick_first strategy.

I'm conscious we have related discussion in #15145 about potentially moving away from the grpc-go experimental resolver so can you please take a look and let me know if you think this pull request is worth merging? If not we can just close no problem.

In my mind reading the issue linked above it sounds like we might still be with grpc-go resolver for a while so I think a bit extra test coverage isn't a bad thing, but I will defer to your judgement here, thanks! 🙏🏻

[ahrtr]

Please also rebase this PR.

[ahrtr]

LGTM

Thanks @jmhbnz