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