postgres_fdw: Stabilize regression test for postgres_fdw_disconnect_all().

The regression test added in commit 411ae64997 caused buildfarm failures.
The cause of them was that the order of warning messages output in the test
was not stable. To fix this, this commit sets client_min_messages to ERROR
temporarily when performing the test generating those warnings.

Per buildfarm failures.

Discussion: https://postgr.es/m/2147113.1611644754@sss.pgh.pa.us

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -9216,15 +9216,15 @@ NOTICE:  drop cascades to 2 other objects
 DETAIL:  drop cascades to user mapping for public on server loopback4
 drop cascades to foreign table ft8
 -- Return false as connections are still in use, warnings are issued.
+-- But disable warnings temporarily because the order of them is not stable.
+SET client_min_messages = 'ERROR';
 SELECT postgres_fdw_disconnect_all();
-WARNING:  cannot close dropped server connection because it is still in use
-WARNING:  cannot close connection for server "loopback" because it is still in use
-WARNING:  cannot close connection for server "loopback2" because it is still in use
  postgres_fdw_disconnect_all 
 -----------------------------
  f
 (1 row)
 
+RESET client_min_messages;
 COMMIT;
 -- Close loopback2 connection and return true.
 SELECT postgres_fdw_disconnect('loopback2');

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -2779,7 +2779,10 @@ SELECT 1 FROM ft8 LIMIT 1;
 SELECT * FROM postgres_fdw_get_connections() ORDER BY 1;
 DROP SERVER loopback4 CASCADE;
 -- Return false as connections are still in use, warnings are issued.
+-- But disable warnings temporarily because the order of them is not stable.
+SET client_min_messages = 'ERROR';
 SELECT postgres_fdw_disconnect_all();
+RESET client_min_messages;
 COMMIT;
 -- Close loopback2 connection and return true.
 SELECT postgres_fdw_disconnect('loopback2');