Fix facedancer unittests

[antoinevg]

This PR is part of:

• Fix ambiguous documentation and implementation for eptri ep_out priming behaviour luna-soc#35
• Fix unit tests when running on Cynthion facedancer#124

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This PR adds some supporting machinery to fix the Facedancer alternate interface unit tests:

• When connecting/disconnecting a USB device pre-existing endpoints from a previous session may still be primed.
• After moondancer receives a set interface request, the underlying eptri ep_out gateware pid state has a 50% chance of being incorrect.
• Added a moondancer method to re-enable the OUT interface for packet reception.

[martinling]

I don't think the SetInterface behaviour is correct here - it should only touch the data toggles on the endpoints of the interface affected.

Since the Moondancer firmware isn't aware of how the endpoints are being used, I think the solution has to be to do this from Python on the host side, by calling the clear_feature_endpoint_halt verb for each affected endpoint when the SetInterface request is being handled.

[martinling]

This looks fine to me and is working in my tests.