release-25.4: roachtest: deflake splits test

[blathers-crl]

Backport 1/1 commits from #170969 on behalf of @pav-kv.

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The test often fails at 200k-290k mark, and once at 174k. Make it target 200k with a bit more time than before.

Fixes-25.2 #170041
Fixes-26.1 #170437
Fixed-26.2 #166202

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Release justification: test deflake

[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 OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are 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. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

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.

[trunk-io]

😎 Merged directly without going through the merge queue, as the queue was empty and the PR was up to date with the target branch - details.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #170989 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR targets solely a file within the 'pkg/cmd/roachtest/' directory, which is explicitly listed as a non-production file pattern under the CockroachDB Backport Policy. This directory is designated for development tools, specifically roachtests, which are used to implement and run tests rather than forming a part of the production code base. Since changes to non-production files are exempt from the typical critical bug justification or feature flag requirements, this PR does not need to adhere to those stipulations. Furthermore, the PR contains a 'Release justification:' line in the body, further supporting its exemption from standard backport requirements because it explicitly states the reason for backporting as 'test deflake'.

✅ ENGREQ Check Passed: No ENGREQ required (non-production code or serious issues).

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

[pav-kv]

/trunk merge