sql/importer: cap metamorphic batch sizes in TestImportCSVStmt

[DrewKimball]

The test has been timing out under metamorphic builds when kv-batch-size and datum-row-converter-batch-size are randomized to their minimum (1). With 18 subtests sharing one cluster, those values blow through the 15-minute package timeout.

Override both to 10 in the test setup. Adds TestingRaiseDefaultKVBatchSize to pkg/sql/row since kv-batch-size had no testing setter; the existing TestingSetDatumRowConverterBatchSize covers the other.

Local timing with both constants forced to 1 vs 10: 74.6s -> 28.2s.

Fixes: #167087
Release note: None

[trunk-io]

😎 Merged successfully - details.

[cockroach-teamcity]

This change is 

[blathers-crl]

Detected infrastructure failure (matched: self-hosted runner lost communication with the server). Automatically rerunning failed jobs. (run link)

[ZhouXing19]

Nice, thanks!! LGTM.

[DrewKimball]

TFTR!

/trunk merge

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #167087: branch-release-26.1, branch-release-26.2.

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[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating backport branch refs/heads/blathers/backport-release-25.4-170013: POST https://api.github.com/repos/DrewKimball/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch 25.4.x failed. See errors above.

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error creating backport branch refs/heads/blathers/backport-release-26.1-170013: POST https://api.github.com/repos/DrewKimball/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch 26.1.x failed. See errors above.

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error creating backport branch refs/heads/blathers/backport-release-26.2-170013: POST https://api.github.com/repos/DrewKimball/cockroach/git/refs: 403 Resource not accessible by integration []

Backport to branch 26.2.x failed. See errors above.

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