SQL and Rust testing guide covering test types, execution, and writing patterns.

• Three primary test formats: .sqltest (preferred for SQL compatibility across backends), TCL .test (legacy, being phased out), and Rust integration tests for regression and complex scenarios
• Run tests via make test for the full suite, make -C testing/sqltests run-cli for SQL tests, or cargo test for Rust tests
• .sqltest format uses simple declarative syntax with @database, test blocks, and expect sections; TCL tests are being converted automatically with the convert command, though manual adjustment of expected results is often needed due to database seed differences
• Every functional change requires a test that fails without the change and passes with it; prefer in-memory databases (:memory:) for test isolation