sqlsmith: add table/index/constraint validation

[mgartner]

Fix up the debt in scrub or build something like SQLite's PRAGMA schema.integrity_check which can validate the state of tables, indexes, and constraints. Then, randomly sprinkle these checks in sqlsmith tests to find bugs in mutations.

Epic: CRDB-2469

Jira issue: CRDB-6370

[mgartner]

Comment from Radu:

The randomized schema change workload must be doing some sort of index integrity checking, I'd investigate what's there

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!