Add tests for coalesce's error semantics

[ggevay]

This is a follow-up to #33103 and #33109, adding tests that show the current behavior.

Note that these tests are not meant to set the current behavior in stone. They are just there to

• let us know if our behavior changes (although we might easily accept changes to it, but it's still better to know about them than to not know about them).
• These tests (together with their comments) are my best guess of what the error semantics specification will mandate for these queries, so writing them down in tests might help in working towards a specification.

(We can incrementally add more tests in this file while we work on error semantics.)

Motivation

Tips for reviewer

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.

[frankmcsherry]

Looks good, and thank you!