mgnp: more integration tests #14

PR #2 added basically one very simple happy-path integration test, where a peer connects to a single service on a remote peer and exchanges a single request/response pair. this doesn't exercise a bunch of the protocol, including potential edge cases.

some other stuff we definitely need to test includes:

• multiple connections to the same service
• multiple connections to different services
• closing a connection and opening a new one (exercising stuff like ID reuse)
• connections reset correctly on closes
• handshake errors (but this isn't actually implemented at all, see mgnp: finish local connection establishment handshakes #12...)