perf: use getQueryResults from DB-API (#375)

I suspect `list_rows` (`tabledata.list`) was being called directly
due to no `page_size` parameter on `QueryJob.result` at the time.

google/cloud/bigquery/dbapi/cursor.py

@@ -229,19 +229,14 @@ def _try_fetch(self, size=None):
             return
 
         if self._query_data is None:
-            client = self.connection._client
             bqstorage_client = self.connection._bqstorage_client
 
             if bqstorage_client is not None:
                 rows_iterable = self._bqstorage_fetch(bqstorage_client)
                 self._query_data = _helpers.to_bq_table_rows(rows_iterable)
                 return
 
-            rows_iter = client.list_rows(
-                self._query_job.destination,
-                selected_fields=self._query_job._query_results.schema,
-                page_size=self.arraysize,
-            )
+            rows_iter = self._query_job.result(page_size=self.arraysize)
             self._query_data = iter(rows_iter)
 
     def _bqstorage_fetch(self, bqstorage_client):

tests/unit/test_dbapi_cursor.py

@@ -66,8 +66,8 @@ def _mock_client(
             num_dml_affected_rows=num_dml_affected_rows,
             dry_run=dry_run_job,
             total_bytes_processed=total_bytes_processed,
+            rows=rows,
         )
-        mock_client.list_rows.return_value = rows
         mock_client._default_query_job_config = default_query_job_config
 
         # Assure that the REST client gets used, not the BQ Storage client.
@@ -102,9 +102,13 @@ def _mock_job(
         num_dml_affected_rows=None,
         dry_run=False,
         total_bytes_processed=0,
+        rows=None,
     ):
         from google.cloud.bigquery import job
 
+        if rows is None:
+            rows = []
+
         mock_job = mock.create_autospec(job.QueryJob)
         mock_job.error_result = None
         mock_job.state = "DONE"
@@ -114,7 +118,7 @@ def _mock_job(
             mock_job.result.side_effect = exceptions.NotFound
             mock_job.total_bytes_processed = total_bytes_processed
         else:
-            mock_job.result.return_value = mock_job
+            mock_job.result.return_value = rows
             mock_job._query_results = self._mock_results(
                 total_rows=total_rows,
                 schema=schema,