Better checks to ensure test data files exist for tables referenced in SQL integration tests

[scholtzan]

For tests defined in https://github.com/mozilla/bigquery-etl/tree/main/tests/sql referenced tables need a schema.json file which will get deployed to the integration test project. Test data is then inserted into these tables and the queries are run against these test tables.

The schema.json files have to exactly match the qualifiers in the SQL query code, otherwise these queries try to query tables in the production environment. This is however currently only caught on main since referenced tables in PRs get deployed to the very same staging environment. So the tests will still succeed.

We should add some additional checks to make sure that any table that is referenced in the query has a matching schema.json file defined.

┆Issue is synchronized with this Jira Task

[scholtzan]

cc @kwindau