Support for async tests

[hberntsen]

The rusty_fork_test! macro does not accept async fn. I need to test some async functions that share global data structures. It would be very useful that this could be done via rusty-fork :).

[mrcnski]

I wouldn't be surprised if there were some tricky corner cases with this. For now the obvious workaround works:

let rt  = tokio::runtime::Runtime::new().unwrap();
rt.block_on(async {
    [...]
})