release-24.1: sql: fix TestRandomSyntaxSchemaChangeDatabase flake

[blathers-crl]

Backport 1/1 commits from #128939 on behalf of @fqazi.

/cc @cockroachdb/release

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Previously, TestRandomSyntaxSchemaChangeDatabase could flake because of contention from multiple Go routines executing SQL against the same database. Some of these schema changes were long-running, making this test unreliable. To address this, this patch creates multiple databases, so that every two connections will share the same database.

Fixes: #128287

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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[cockroach-teamcity]

This change is