Capture output from PostgreSQL notice receiver in a test

activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb

@@ -521,10 +521,14 @@ module ActiveRecord
 
       def test_ignores_warnings_when_behaviour_ignore
         with_db_warnings_action(:ignore) do
+          # libpq prints a warning to stderr from C, so we need to stub
+          # the whole file descriptors, not just Ruby's $stdout/$stderr.
+          _out, err = capture_subprocess_io do
             result = @connection.execute("do $$ BEGIN RAISE WARNING 'foo'; END; $$")
-
             assert_equal [], result.to_a
           end
+          assert_match(/WARNING:  foo/, err)
+        end
       end
 
       def test_logs_warnings_when_behaviour_log