pytest: make run_handshake handle unsolicited Block messages

[mina86]

This addresses the following failure:

Traceback (most recent call last):
  File "tests/spec/network/peers_request.py", line 44, in <module>
    asyncio.run(main())
  File "/usr/lib/python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.7/asyncio/base_events.py", line 579, in run_until_complete
    return future.result()
  File "tests/spec/network/peers_request.py", line 35, in main
    await run_handshake(conn1, nodes[0].node_key.pk, key_pair_1, listen_port=12346)
  File "lib/peer.py", line 182, in run_handshake
    assert response.enum == 'Handshake', response.enum if response.enum != 'HandshakeFailure' else response.HandshakeFailure[1].enum
AssertionError: Block

Fixes: #4432

[bowenwang1996]

This is weird. Handshake is the first message we exchange when establishing a connection. Therefore this implies that the test may not intercept the messages correctly. cc @mfornet @pmnoxx

[mina86]

I think what happened was that between flagging handshake as finished and sending handshake to the pear, another thread notice the new peer and sent the block. The flow of the handshake is that it first goes to Peer which then sends Consolidate to PeerManager which registers a new peer and responds back to Peer with an accept and only then Peer sends handshake back to the other node.