test: introduce libp2p-swarm-test

This patch-set introduces `libp2p-swarm-test`. It provides utilities for quick and safe bootstrapping of tests for `NetworkBehaviour`s. The main design features are:

- Everything has timeouts
- APIs don't get in your way
- Minimal boilerplate

Closes #2884.

Pull-Request: #2888.

protocols/gossipsub/src/protocol.rs

@@ -563,7 +563,7 @@ mod tests {
     use crate::Behaviour;
     use crate::IdentTopic as Topic;
     use libp2p_core::identity::Keypair;
-    use quickcheck::*;
+    use quickcheck_ext::*;
 
     #[derive(Clone, Debug)]
     struct Message(RawMessage);