nextcloud: Don't wait for container to be killed

packages/nextcloud/test/helper.dart

@@ -36,12 +36,14 @@ class DockerContainer {
     }
   }
 
-  Future destroy() => runExecutableArguments(
+  void destroy() => unawaited(
+        runExecutableArguments(
           'docker',
           [
             'kill',
             id,
           ],
+        ),
       );
 
   Future<String> collectLogs() async {