Fix tests: Don't try to use policykit during the tests

Creating a policykit proxy was causing timeouts for me for unclear
reasons when running under the tests. But we're not using policykit at
all during the tests, so we can just avoid this call completely.

system-helper/flatpak-system-helper.c

@@ -783,11 +783,14 @@ main (int    argc,
   if (verbose)
     g_log_set_handler (G_LOG_DOMAIN, G_LOG_LEVEL_DEBUG, message_handler, NULL);
 
-  authority = polkit_authority_get_sync (NULL, &error);
-  if (authority == NULL)
+  if (!on_session_bus)
     {
-      g_printerr ("Can't get polkit authority: %s\n", error->message);
-      return 1;
+      authority = polkit_authority_get_sync (NULL, &error);
+      if (authority == NULL)
+        {
+          g_printerr ("Can't get polkit authority: %s\n", error->message);
+          return 1;
+        }
     }
 
   exe_path_len = readlink ("/proc/self/exe", exe_path, sizeof (exe_path) - 1);