Feature request: async tests

Currently we generate tests like:

```rust
#[test]
fn test() { ... }
```

It would also be useful to support asynchronous tests of the form:

```rust
#[tokio::test]
async fn test() { ... }
```

For this we need three things:

- stop adding `#[test]` automatically
- let the user specify the `#[tokio::test]` attribute themselves (once #10 is merged)
- add the `async` prefix to each `fn`
