The loopback functionality was never implemented, not for regular bulk
endpoints. By adding it, and adding pairs of endpoints, we can easily
catch buffer management problems. These tests currently fail on
st_usbfs devices.
This did require renumbering the endpoints, as dwc_otg_fs only offers
three endpoints in each direction, and they can't be arbitrary numbers,
unlike on st_usbfs.
See https://github.com/libopencm3/libopencm3/pull/880 and related tickets.
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