[testing/bdd] async `describe`

**Is your feature request related to a problem? Please describe.**

in vitest we can use `async` functions as a callback to `describe()`, which allows setting up group-related resource, e.g.:

```ts
describe('foo', async () => {
const foo = await createFoo()
afterAll(() => foo.destroy())

// ... tests ...
})
```

however `describe` currently doesn't allow async functions to be passed

**Describe the solution you'd like**

support async `describe` callbacks

**Describe alternatives you've considered**

- move all `await`s to top level - the downside is that group-related stuff stops being grouped
- initialize everything with a `beforeAll` hook - the downside is that we have to use non-null assertions for typescript + increases boilerplate