release-24.1: kvstreamer: speed up TestStreamerVaryingResponseSizes

[blathers-crl]

Backport 1/1 commits from #123030 on behalf of @yuzefovich.

/cc @cockroachdb/release

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We just saw a test timeout due to BatchBytesLimit randomization, so this commit disables batch randomizations for TestStreamerVaryingResponseSizes.

Fixes: #123007.

Release note: None

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Release justification: test-only change.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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  here).
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backport.

[cockroach-teamcity]

This change is 

[yuzefovich]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball and @mgartner)