Bump @vue/test-utils from 1.0.0-beta.27 to 1.0.0-beta.28

Bumps [@vue/test-utils](https://github.com/vuejs/vue-test-utils) from 1.0.0-beta.27 to 1.0.0-beta.28.
- [Release notes](https://github.com/vuejs/vue-test-utils/releases)
- [Changelog](https://github.com/vuejs/vue-test-utils/blob/dev/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-test-utils/compare/v1.0.0-beta.27...v1.0.0-beta.28

)

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