Fix flaky connection shutdown test

tests/test_server.cpp

@@ -61,6 +61,7 @@ struct ShutdownTestHandler : ConnectionHandler {
   std::span<std::string_view> reply;
   WeakRef<MessageSender> wconn;
   std::latch received_data{1};
+  std::latch connection_closed_latch{1};
   ConnectionShutdown shutdown_mode = ConnectionShutdown::None;
   std::atomic<bool> connection_closed{false};
 
@@ -71,7 +72,10 @@ struct ShutdownTestHandler : ConnectionHandler {
     received_data.count_down();
   }
 
-  void on_connection_closed(Connection &) override { connection_closed = true; }
+  void on_connection_closed(Connection &) override {
+    connection_closed = true;
+    connection_closed_latch.count_down();
+  }
 };
 
 TEST_CASE("Connection shutdown write-only mode") {
@@ -159,9 +163,9 @@ TEST_CASE("Connection shutdown full mode") {
   int close_result = read(fd, extra_buf, 1);
   CHECK(close_result == 0); // EOF indicates connection was closed
 
-  // The blocking read already synchronized - connection is definitely closed
-  CHECK(handler.connection_closed.load() ==
-        true); // Connection should be closed
+  // Wait for connection closed callback to be called
+  handler.connection_closed_latch.wait();
+  CHECK(handler.connection_closed.load() == true);
 
   close(fd);
   server->shutdown();