Move more files around.

test/runner/engine_test.dart

 // Copyright (c) 2015, 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:unittest/src/backend/declarer.dart';
 import 'package:unittest/src/backend/state.dart';
 import 'package:unittest/src/backend/suite.dart';
 import 'package:unittest/src/runner/engine.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 67 matching lines @@
     var engine = new Engine([new Suite("suite", declarer.tests)]);
     expect(engine.run(), completion(isFalse));
   });
 
   test(".run() may not be called more than once", () {
     var engine = new Engine([]);
     expect(engine.run(), completes);
     expect(() => engine.run(), throwsStateError);
   });
 }