refactor integration_tests_sv2::{MockDownstream, MockUpstream}

[plebhash]

close #147

blocker to #146

• removes MockDownstream::next_message_from_upstream (this can be achieved with a Sniffer)
• removes MockUpstream::next_message_from_downstream (this can be achieved with a Sniffer)
• MockUpstream no longer "responds" to received messages
• {MockUpstream,MockDownstream}::start return Sender<AnyMessage> (instead of Sender<MessageFrame>)
• test_mock_upstream + test_mock_downstream unified into a single test_mock_roles to reduce redundancy

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this does NOT address #148

[plebhash]

cc @xyephy

[xyephy]

cc @xyephy

I will have a look at it

[xyephy]

LGTM, tested the PR seems to pass all tests.