docs: Missing Vue import (#1445)

* Missing Vue import

`import Vue` was missing in the code block.
when someone, new to vue tries to follow the guide, it becomes a pain-point to figure why test case which included `Vue.nextTick()` failed.

* Update testing-async-components.md

Author: quriosapien

Diff for: docs/guides/testing-async-components.md

@@ -16,6 +16,12 @@ In practice, this means you have to use `Vue.nextTick()` to wait until Vue has p
 The easiest way to use `Vue.nextTick()` is to write your tests in an async function:
 
 ```js
+// import Vue at the top of file
+import Vue from 'vue'
+
+// other code snippet...
+
+// inside test-suite, add this test case
 it('button click should increment the count text', async () => {
   expect(wrapper.text()).toContain('0')
   const button = wrapper.find('button')