test/hil: run the printer write in a child, like the read

test_device_printer_to_cdc opened /dev/usb/lp* on the worker itself and let
hil_util.bounded_open abandon a thread when the open blocked. usblp allows one
opener -- usblp_open() returns -EBUSY while usblp->used (v6.12.96 usblp.c) --
so the abandoned thread's fd poisoned the node for every later test that worker
ran. The read half already avoided this by forking; the write half now does too,
via the same run_alongside, and a killed child takes its fd with it.

This removes the only production caller of bounded_open.