Add regression test for watch dropping event on compaction

[serathius]

What would you like to be added?

Spinoff from #18089

Should add a regression test that can be called via make test-robustness-issue18089 like in https://github.com/etcd-io/etcd/pull/17680/files.

Why is this needed?

Allow us to validate that issues like #18089 can be reproduced in future

[jmao-dd]

@serathius can I take a stab at it? I am new to this project and this seems to be a good first issue.

[serathius]

There might be simpler tasks to take, still feel free to take a stab on it. If needed please reach out to me on slack #sig-etcd channel on Kubernetes (get an invite)

[jmao-dd]

@serathius here is some idea I have got to implement this test. Please let me know if I am on the right track.