release-23.2: roachtest: add backup-restore/small-ranges

[blathers-crl]

Backport 1/1 commits from #112356 on behalf of @msbutler.

/cc @cockroachdb/release

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This patch adds a new backup-restore roachtest variant that reduces default
range size for user databases in the backup-restore/round-trip roachtest to
simulate a larger cluster, in terms of range count, at smaller data sizes. In
addition, the roachtest scales down a few cluster settings such that the ratio
of rangeSize/clusterSetting remains constant.

This patch should allow us to recreate a roachtest workload that can simulate
the conditions that lead to #109483 but at smaller data sizes.

Informs #109483

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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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is