Remove flaky flutter_tester test (#18916)

This test was designed to ensure flutter_tester keeps running (previously it would quit immediately). However it's turned out ot be rather flaky and we have new tests on the way that supersede this by actually testing real things (debug stepping, reloading, expression evaluation).

packages/flutter_tools/test/integration/flutter_tester_test.dart

@@ -74,52 +74,5 @@ void main() {
 
       expect(await device.stopApp(null), isTrue);
     });
-
-    testUsingContext('keeps running', () async {
-      writePubspec(tempDir.path);
-      writePackages(tempDir.path);
-      await getPackages(tempDir.path);
-
-      final String mainPath = fs.path.join('lib', 'main.dart');
-      writeFile(mainPath, r'''
-import 'package:flutter/material.dart';
-
-void main() => runApp(new MyApp());
-
-class MyApp extends StatelessWidget {
-  @override
-  Widget build(BuildContext context) {
-    return new MaterialApp(
-      title: 'Flutter Demo',
-      home: new Container(),
-    );
-  }
-}
-''');
-
-      // Capture process output so that if the process quits we can print the
-      // stdout/stderr in the error.
-      final StringBuffer logs = new StringBuffer();
-      device.getLogReader().logLines.listen(logs.write);
-
-      final LaunchResult result = await start(mainPath);
-
-      expect(result.started, isTrue);
-      expect(result.observatoryUri, isNotNull);
-
-      // This test has been seen to flake on mac_bot because the process did not keep running.
-      // and on Travis with a timeout:
-      // 05:33 +416 ~9 -1: test/integration/flutter_tester_test.dart: FlutterTesterDevice start [E]                                                                                                             
-      // TimeoutException after 0:00:30.000000: Test timed out after 30 seconds.
-      // package:test  Invoker._onRun.<fn>.<fn>.<fn>
-      // 06:00 +623 ~9 -1: Some tests failed.
-      //
-      // TODO(dantup): Find a way to better log what's going on before un-skipping again.
-
-      await new Future<void>.delayed(const Duration(seconds: 3));
-      expect(device.isRunning, true, reason: 'Device did not remain running.\n\n$logs'.trim());
-
-      expect(await device.stopApp(null), isTrue);
-    }, skip: true);
   });
 }