Don't try to stop the same app twice in flutter_attach test (#20905)

This test is sometimes flaky (#20822) because the two processes try to stop the same app. This fix changes to just gracefully terminate the attach process without explicitly trying to stop and then uses the original spawning process to stop the app.

I can't repro the flake locally to be certain, but I've verified only one stop command is sent now so it *should* be good.

packages/flutter_tools/test/integration/flutter_attach_test.dart

@@ -24,8 +24,11 @@ void main() {
   });
 
   tearDown(() async {
+    // 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.stop();
     tryToDelete(tempDir);
   });
 

packages/flutter_tools/test/integration/test_driver.dart

@@ -179,6 +179,8 @@ class FlutterTestDriver {
     return _proc.exitCode.timeout(quitTimeout, onTimeout: _killGracefully);
   }
 
+  Future<int> quit() => _killGracefully();
+
   Future<int> _killGracefully() async {
     if (_procPid == null)
       return -1;