tests: 🦹 integrate mock consensus engine with `proptest`

[cratelyn]

stub issue:

once we are finished with #3588, we should consider how/whether to try integrating this with something proptest. defining a "strategy" for polling the engine in different ways would allow us to exercise our state machine components more extensively.

this should be revisit at a later point, when we have a mock consensus engine and some hand-written tests exercising an IBC handshake, for example.