These changes make the tests work with the coming `codeql test` support.
The `queries.xml` file defines which extractor the `codeql test`
runner will use to extract databases for the tests. In the future one
will be able to write this information in `qlpack.yml`, but we can't
do that immediately because the _existing_ CodeQL tooling would refuse
to parse a `qlpack.yml` that has the new field in it.
