feat(karma integration): add a file to init specs to improve karma integration

Since the standard auto initialization of specs does not work with Karma, you had to call `guinness.initSpecs()` manually.

Now, you can just include the init file, as follows:

    files: [
      "test/main1_test.dart",
      "test/main2_test.dart",
      "packages/guinness/init_specs.dart",
      {pattern: '**/*.dart', watched: true, included: false, served: true}
    ]