Write the very first async test

[programarivm]

It'd be awesome if a suite of async tests could be written using amphp/phpunit-util. The thing though is that at the moment the newest version of AMPHP can't be installed on our environment because of rubix/ml 2.4.0 using a non-compatible version of amphp/parallel and amphp/byte-stream. See amphp/websocket-server#27