Add code to locate a plugin within a package

pkg/analysis_server/test/src/plugin/test_all.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
+import 'plugin_locator_test.dart' as plugin_locator_test;
 import 'result_collector_test.dart' as result_collector_test;
 
 main() {
   defineReflectiveSuite(() {
+    plugin_locator_test.main();
     result_collector_test.main();
   });
 }