fix: support handshake-only with no muxers (#519)

Allows transports such as WebRTC-Direct to skip the early muxer
negotiation.

This handles an edge case where you have a libp2p node
configured with no dedicated muxer(s), instead transport(s) that
incorporate their own muxers but that use Noise to exchange PeerIDs.

Splits the existing muxer negotiation test out into it's own file
and adds more tests to handle the various configurations.

Author: achingbrain