Add loom testing for async usage of Receiver

Currently Tokio can't be build with loom support. But I started working on a loom aware async executor: https://github.com/faern/loom-executor. It's currently in a messy state and does not work well. But at some point it would be nice to have loom validate the correctness of the `Future` implementation on the `Receiver`