qgen: add proof of concept infra to build Query() objects for testing

To test IMPALA-3993, we need static Query() objects as test data. It's a bit cumbersome to build Query() objects by hand, because the objects get set over time as the query generator logic path occurs. This is a different use case: we know a priori what we want our queries to look like. This means we need to be able to build them up rather cleanly and conveniently.