Update bot configuration to use test package

R=sgjesse@google.com

Review URL: https://codereview.chromium.org//1318763002 .

.status

-# 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.
-
-# Skip non-test files ending with "_test".
-packages/*: Skip
-*/packages/*: Skip
-*/*/packages/*: Skip
-*/*/*/packages/*: Skip
-*/*/*/*packages/*: Skip
-*/*/*/*/*packages/*: Skip
-

.test_config

+{
+  "test_package": true
+}
\ No newline at end of file