test: fix racy loop-variable capture in SubscriptionThreadSafety

- the subscriber threads captured the loop counter by reference while
  the spawning loop kept incrementing it: a genuine data race
- depending on timing, threads could also end up with duplicate
  subscriber names; capture the counter by value instead

test/plugin_services/_control.cxx

@@ -156,7 +156,7 @@ TEST_F(PluginServices, SubscriptionThreadSafety)
     std::array<std::unique_ptr<std::thread>, subscribers> threads;
     auto id = 0;
     for (auto& thread : threads) {
-        thread = std::make_unique<std::thread>([&](){
+        thread = std::make_unique<std::thread>([&, id](){
             auto const subscriber = fair::mq::tools::ToString("subscriber_", id);
             for (auto i = 0; i < attempts; ++i) {
                 mServices.SubscribeToDeviceStateChange(subscriber, [](DeviceState){});