tests: Ensure ctrlport responses come in expected order

Due to thread scheduling on the test OS, requests made in close
succession could be responded to out of order, which does not correctly
model the hardware. This change ensures that responses come in the same
order they were requested.

host/tests/ctrlport_endpoint_test.cpp

@@ -93,6 +93,8 @@ public:
     std::mutex response_threads_mutex{};
     std::vector<std::thread> response_threads{};
     std::atomic<size_t> pending_responses{0};
+    std::atomic<size_t> response_sequence{0};
+    std::atomic<size_t> responses_delivered{0};
 
     ctrlport_endpoint::send_fn_t send_fn;
     mock_clock_iface client_clk;
@@ -219,12 +221,18 @@ private:
     void schedule_async_response(const ctrl_payload& response, int64_t delay_ms = 1)
     {
         pending_responses++;
+        // Ensure responses are delivered in order by waiting on the previous one
+        size_t my_seq = response_sequence++;
 
         std::lock_guard<std::mutex> lock(response_threads_mutex);
-        response_threads.emplace_back([this, response, delay_ms]() {
+        response_threads.emplace_back([this, response, delay_ms, my_seq]() {
             // Small delay to simulate hardware response time
             std::this_thread::sleep_for(std::chrono::milliseconds(delay_ms));
+            while (responses_delivered.load() < my_seq) {
+                std::this_thread::yield();
+            }
             endpoint->handle_recv(response);
+            responses_delivered++;
             pending_responses--;
         });
     }