USB: testusb: add path /dev/bus/usb to default search paths for usbfs

As real device-nodes managed by udev whose nodes lived in /dev/bus/usb
are mostly used today, let testusb tool use that directory as one default
path make tool be more convenient to use.

Signed-off-by: Du Changbin <changbinx.du@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>