Add missing unit tests for `# noqa: A`-like cases (#16677)
## Summary
Follow-up to #16659.
This change adds tests for these three cases, which are (also) not
covered by existing tests:
* `# noqa: A` (lone incomplete code)
* `# noqa: A123, B` (complete codes, last one incomplete)
* `# noqa: A123B` (squashed codes, last one incomplete)
