Introspection engine tests

There are two entry points for introspection tests. The tests involving writing plain Rust tests, in introspection_tests.rs, and the simple test setup, in simple.rs.

The simple test setup

Many test cases only involve setting up a database schema with SQL, then introspecting that database. We want to keep this kind of tests as declarative as possible.

The so-called simple test suite consists of single SQL files, that will be run as-is. In addition to the SQL content, comments are used for configuration and expectations.

All the SQL files under the tests/simple directory and all its subdirectories will be run, in parallel and in no particular order.

The SQL test file must start with a comment specifying which tags the test applies to. It may be followed by a line specifying which tags to exclude. The tags are the same as for the regular test setup.

Example:

-- tags=postgres
-- exclude=cockroachdb

The file must also end with a block SQL comment (/*\n ... \n*/\n). This block comment contains the expected introspected schema.

Like expect_test tests, the last block comment can be left empty at first, and updated like a snapshot test using the UPDATE_EXPECT=1 env var.