release-24.3: sql/schemachanger: deflake TestPause.*

[fqazi]

Backport 1/1 commits from #161884 on behalf of @fqazi.

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Previously, these tests could timeout, because they had a short time interval to wait for schema changes to complete (the default of 45 seconds). This patch makes the pause tests wait for completion before checking the final status.

Fixes: #161743
Fixes: #161742
Fixes: #161693
Fixes: #161456

Release note: None

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Release justification: test only change

[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]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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

[blathers-crl]

✅ PR #161893 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request is compliant with the backport policy for the following reasons: 1. It contains a 'Release justification: test only change' in the PR body, which explicitly indicates that this is a test-only modification. According to the CockroachDB backport policy, changes that are solely for non-production, test-only files are exempt from standard backport policy requirements including the need for the change fixing a critical bug or being gated behind a feature flag. 2. The modified file 'pkg/sql/schemachanger/sctest/cumulative.go' fits the pattern of test files (e.g., files in test scenarios or infrastructure). Consequently, the changes are deemed non-production, supporting the test-only exemption.

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

[cockroach-teamcity]

This change is