Clariy unit testing resources (#1470)

Author: sdras

src/v2/guide/unit-testing.md

@@ -130,3 +130,5 @@ it('updates the rendered message when vm.message updates', done => {
 ```
 
 We are planning to work on a collection of common test helpers to make it easier to render components with different constraints (e.g. shallow rendering that ignores child components) and assert their output.
+
+For more in-depth information on unit testing in vue, check out [vue-test-utils](https://vue-test-utils.vuejs.org/en/) and our cookbook entry about [unit testing vue components](https://vuejs.org/v2/cookbook/unit-testing-vue-components.html).