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