This PR implements conformance tests between our network backends
(litep2p and libp2p).

The PR creates a setup for extending testing in the future, while
implementing the following tests:
- connectivity check: Connect litep2p -> libp2p and libp2p -> litep2p
- request response check: Send 32 requests from one backend to the other
- notification check: Send 128 ping pong notifications and 128 from one
backend to the other

cc @paritytech/networking

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>