Fix raw_server_socket_cancel_test on macos by always call close on done.

tests/standalone/io/raw_server_socket_cancel_test.dart

Index: tests/standalone/io/raw_server_socket_cancel_test.dart
diff --git a/tests/standalone/io/raw_server_socket_cancel_test.dart b/tests/standalone/io/raw_server_socket_cancel_test.dart
index b32b535f7a2dfa7636ed36c4a6db48a931fb6bd2..a927c7cd02dd0dc939ac2cd034f0ab5483babc6e 100644
--- a/tests/standalone/io/raw_server_socket_cancel_test.dart
+++ b/tests/standalone/io/raw_server_socket_cancel_test.dart
@@ -26,6 +26,9 @@ void testCancelResubscribeServerSocket(int socketCount, int backlog) {
     void checkDone() {
       if (doneCount + errorCount + earlyErrorCount == socketCount) {
         port.close();
+        // Be sure to close as subscription.cancel may not be called, if
+        // backlog prevents acceptCount to grow to socketCount / 2.
+        socket.close();
       }
     }