add test case when raise ServiceUnavailable in is_retryable (#1224)

* raise ServiceUnavailable and test that it is retry-able

---------

Co-authored-by: Mike Alfare <[email protected]>

Author: jx2lee

.changes/unreleased/Features-20240505-011838.yaml

@@ -0,0 +1,6 @@
+kind: Features
+body: add is_retryable test case when raise ServiceUnavailable
+time: 2024-05-05T01:18:38.737882+09:00
+custom:
+  Author: jx2lee
+  Issue: "682"

tests/unit/test_bigquery_connection_manager.py

@@ -84,12 +84,14 @@ def test_is_retryable(self):
         rate_limit_error = exceptions.Forbidden(
             "code broke", errors=[{"reason": "rateLimitExceeded"}]
         )
+        service_unavailable_error = exceptions.ServiceUnavailable("service is unavailable")
 
         self.assertTrue(_is_retryable(internal_server_error))
         self.assertTrue(_is_retryable(bad_request_error))
         self.assertTrue(_is_retryable(connection_error))
         self.assertFalse(_is_retryable(client_error))
         self.assertTrue(_is_retryable(rate_limit_error))
+        self.assertTrue(_is_retryable(service_unavailable_error))
 
     def test_drop_dataset(self):
         mock_table = Mock()