usbtest: confine testusb's device scan to the DUT node (#4049)

* usbtest: confine testusb's device scan to the DUT node

Stock testusb ftw()s and opens every usbfs node even with -D, so one
peer stuck in enumeration held a lock that timed out every other
board's cases. -A <node> makes glibc ftw() visit only that node.

Fixes #4047

* usbtest.py: simplify
6 files changed