nixos/tests/prometheus: wait for influxdb exporter before quering

Previously the influxdb exporter test was flaky as even after the
service has started there is still a race before the service is actually
listening and accepting connection on port 9122.

With this commit the test will wait for the port to be open before
proceeding.

nixos/tests/prometheus-exporters.nix

@@ -280,6 +280,7 @@ let
       };
       exporterTest = ''
         wait_for_unit("prometheus-influxdb-exporter.service")
+        wait_for_open_port(9122)
         succeed(
           "curl -XPOST http://localhost:9122/write --data-binary 'influxdb_exporter,distro=nixos,added_in=21.09 value=1'"
         )