The list_devices API has a race condition where sometimes it will
return zero or less than the expected number of devices and fail
the test. A fix is in place for when only a single devices is
expected, but some tests expect more than one device. This changes
wait_to_appear to an integer, and the caller can specify the number
of devices they expect to get back. The default stays as it was,
zero or "return cached devices".
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