nixos/tests/prometheuts-exporters.unpoller: fix test script

Unpoller behaviour changed and now it'll not server metrics if no
controller available, so, let's check for not available controller
message instead.

nixos/tests/prometheus-exporters.nix

@@ -1244,10 +1244,8 @@ let
       exporterConfig.enable = true;
       exporterConfig.controllers = [{ }];
       exporterTest = ''
-        wait_for_unit("prometheus-unpoller-exporter.service")
-        wait_for_open_port(9130)
-        succeed(
-            "curl -sSf localhost:9130/metrics | grep 'unpoller_build_info{.\\+} 1'"
+        wait_until_succeeds(
+            'journalctl -eu prometheus-unpoller-exporter.service -o cat | grep "Connection Error"'
         )
       '';
     };