release-25.1: changefeedccl: fix TestNoBackfillAfterNonTargetColumnDrop

[andyyang890]

Backport 2/2 commits from #150215.

/cc @cockroachdb/release

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This patch fixes TestNoBackfillAfterNonTargetColumnDrop so that it
actually checks that no backfills occur after a non-target column drop.

Fixes #150214

Release note: None

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Release justification: test-only fix

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

✅ PR #150286 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR changes a file changefeed_test.go which is a test file, indicated by the _test.go suffix. According to the CockroachDB backport policy, changes that only affect non-production code, tests, or development tools are exempt from the critical bug justification or feature flag requirements. Additionally, the PR contains a release justification stating it is a 'test-only fix', which aligns with the policy for test-only changes not being subject to the usual requirements.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is