Swap order of attach/stop in flutter_attach integration test (#21106)

This is a bit of a stab in the dark for a flake sometimes seen on bots (but I can't repro locally) #20822. Detaching from a Flutter app currently seems to terminate it, so it may be the cause of the "app not found" when trying to stop the app subsequently.

This change means we stop the app from the main process first, before terminating the attach process.

packages/flutter_tools/test/integration/flutter_attach_test.dart

@@ -27,8 +27,8 @@ void main() {
     // We can't call stop() on both of these because they'll both try to stop the
     // same app. Just quit the attach process and then send a stop to the original
     // process.
-    await _flutterAttach.quit();
     await _flutterRun.stop();
+    await _flutterAttach.quit();
     tryToDelete(tempDir);
   });