test/cluster: mark test-constant-sink as a regression test

[teskje]

This test previously documented the current wrong behavior. It looks like that behavior has since been fixed by #31152, and this is a regression test instead. Update its documentation accordingly.

Motivation

• This PR fixes a recognized bug.

Closes https://github.com/materializeInc/database-issues/issues/8842

Tips for reviewer

This test is known to be flaky: https://github.com/MaterializeInc/database-issues/issues/9933. There is no reason to believe that this change fixes the flake.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.