function testExec_ValidProjectionList_GroupBy() {
  var e = db.getSchema().getEmployee();
  var query = new lf.query.SelectBuilder(
      hr.db.getGlobal(), [e.jobId, lf.fn.avg(e.salary)]);
  return query.from(e).groupBy(e.jobId, e.departmentId).exec();
}
Tests that constructing queries involving Select#groupBy() succeed if a
valid combination of projection and groupBy list is requested. This test
checks that columns in groupBy() does not necessarily exist in projection
list.
@return {!IThenable}