tests: harden transient-connectivity retries for post-restart queries

- has_needed_replication_slots(): bump retry_timeout 5s -> 15s, the
  existing 0.5s-backoff retry loop wasn't always covering a keeper-
  triggered Postgres restart under a loaded CI runner
  (test_multi_ifdown.py::test_003_add_standby saw a bare
  Connection refused).
- test_citus_multi_standbys.py: use the existing run_sql_query_retry()
  helper for worker2c instead of a bare run_sql_query(), which had no
  retry at all (test_003_002_all_workers_have_data saw the same).

Author: dimitri

tests/pgautofailover_utils.py

@@ -1829,7 +1829,7 @@ def list_replication_slot_names(self):
             self.print_debug_logs()
             raise e
 
-    def has_needed_replication_slots(self, retry_timeout=5):
+    def has_needed_replication_slots(self, retry_timeout=15):
         """
         Each node is expected to maintain a slot for each of the other nodes
         the primary through streaming replication, the secondary(s) manually

tests/test_citus_multi_standbys.py

@@ -192,7 +192,7 @@ def test_003_002_all_workers_have_data():
     results = worker2b.run_sql_query(q1)
     eq_(results, r2)
 
-    results = worker2c.run_sql_query(q1)
+    results = worker2c.run_sql_query_retry(q1)
     eq_(results, r2)