Remove flutter_tester test (#21493)

I'm working on fixing up all tests that rely on fs.currentDirectory to allow us to run concurrently. This one proved to be really tricky because it calls a lot of code and spawns multiple processes that have a mix of absolute and relative paths passed to them. I managed to handle some of it with a ChrootFileSystem, but the paths passed to the external processes were wrong.

The functionality here is covered by all of the integration tests that use flutter_tester that were added after this, including flutter_run_test.dart, lifetime_test.dart etc. in this same folder but because they're run through `flutter run` as a separate process they get to set their own working directories without affecting other tests if run concurrently.

packages/flutter_tools/test/integration/flutter_tester_test.dart

-// Copyright 2018 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'dart:async';
-
-import 'package:file/file.dart';
-import 'package:flutter_tools/src/base/file_system.dart';
-import 'package:flutter_tools/src/build_info.dart';
-import 'package:flutter_tools/src/device.dart';
-import 'package:flutter_tools/src/tester/flutter_tester.dart';
-
-import '../src/common.dart';
-import '../src/context.dart';
-import 'test_utils.dart';
-
-void main() {
-  Directory tempDir;
-  Directory oldCurrentDir;
-
-  setUp(() async {
-    tempDir = fs.systemTempDirectory.createTempSync('flutter_tester_device_test.');
-    oldCurrentDir = fs.currentDirectory;
-    fs.currentDirectory = tempDir;
-  });
-
-  tearDown(() {
-    fs.currentDirectory = oldCurrentDir;
-    tryToDelete(tempDir);
-  });
-
-  group('FlutterTesterDevice', () {
-    FlutterTesterDevice device;
-
-    setUp(() {
-      device = new FlutterTesterDevice('flutter-tester');
-    });
-
-    Future<LaunchResult> start(String mainPath) async {
-      return await device.startApp(
-        null,
-        mainPath: mainPath,
-        debuggingOptions: new DebuggingOptions.enabled(
-          const BuildInfo(BuildMode.debug, null),
-        ),
-      );
-    }
-
-    testUsingContext('start', () async {
-      writePubspec(tempDir.path);
-      writePackages(tempDir.path);
-
-      final String mainPath = fs.path.join('lib', 'main.dart');
-      writeFile(mainPath, r'''
-import 'dart:async';
-void main() {
-  new Timer.periodic(const Duration(milliseconds: 1), (Timer timer) {
-    print('Hello!');
-  });
-}
-''');
-
-      final LaunchResult result = await start(mainPath);
-      expect(result.started, isTrue);
-      expect(result.observatoryUri, isNotNull);
-
-      final String line = await device.getLogReader().logLines.firstWhere((String line) => !line.contains('TeXGyreSchola'));
-      expect(line, equals('Hello!'));
-
-      expect(await device.stopApp(null), isTrue);
-    });
-  });
-}