release-24.3: changefeedccl: fix test flake by waiting for rangefeed start

[aerfrei]

Backport 1/1 commits from #152689 on behalf of @aerfrei.

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Previously, this test would flake because a table that we were asserting the admin user could see hadn't yet become visible. Now we assert the changefeed and rangefeed have started by waiting for a highwater mark.

Epic: none
Fixes: #152060

Release note: None

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Release justification: Test only change in response to test flake

[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 #155191 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request modifies a test file pkg/ccl/changefeedccl/changefeed_test.go, which is classified under non-production code. Since only test files are affected, this PR falls within the non-production changes exception of the backport policy. Furthermore, the PR body includes a 'Release justification:' line stating 'Test only change in response to test flake', which specifies that this is an exempt change due to it being a response to test instability.

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

[cockroach-teamcity]

This change is