docs: Update async-suspense.md (#2034)

* Update async-suspense.md

Without flushPromises() after mounting an async component, the wrapper will be empty.

* Update async-suspense.md

Author: tobi-fis

docs/guide/advanced/async-suspense.md

@@ -129,21 +129,23 @@ const Async = defineComponent({
 must be tested as follow:
 
 ```js
-test('Async component', () => {
+test('Async component', async () => {
   const TestComponent = defineComponent({
     components: { Async },
     template: '<Suspense><Async/></Suspense>'
   })
 
   const wrapper = mount(TestComponent)
+  await flushPromises();
   // ...
 })
 ```
+Note: To access your `Async` components' underlying `vm` instance, use the return value of `wrapper.findComponent(Async)`. Since a new component is defined and mounted in this scenario, the wrapper returned by `mount(TestComponent)` contains its' own (empty) `vm`.
 
 ## Conclusion
 
 - Vue updates the DOM asynchronously; tests runner executes code synchronously instead.
 - Use `await nextTick()` to ensure the DOM has updated before the test continues.
 - Functions that might update the DOM (like `trigger` and `setValue`) return `nextTick`, so you need to `await` them.
 - Use `flushPromises` from Vue Test Utils to resolve any unresolved promises from non-Vue dependencies (such as API requests).
-- Use `Suspense` to test components with an asynchronous `setup`.
+- Use `Suspense` to test components with an asynchronous `setup` in an asynchronous test function.