release-24.3: 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.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #150287 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR is compliant with the CockroachDB backport policy due to its nature as a test-only fix. The changes are limited to a file named changefeed_test.go, which matches the non-production file pattern *_test.go. According to the backport policy, changes that affect only non-production code, specifically test files, are exempt from the standard critical bug and feature flag requirements. Additionally, the PR body includes a 'Release justification: test-only fix' line, explicitly noting that its purpose is for addressing a test-specific issue rather than altering production behavior.

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