test/hil: run the HID echo in a child, which is the only bound that works (#3852)HEADmaster

hid_generic_inout was the last unbounded blocking IO in the file. hidapi's
hidraw backend reads manufacturer/product via udev for each device reaching
create_device_info_for_device, both usb_string_attr served under the device
lock a wedged usbfs ioctl holds — and every DUT here is VID cafe, so a wedged
sibling stalls the walk.

A thread cannot bound it: cython-hidapi calls hid_open and hid_close bare
(0.15.0 hid.pyx), so they hold the GIL and the waiter can never resume.
Measured — a 1.0s bound never returned. run_cmd's killpg reaches a child
regardless; it gains an argv form for the -c body.

Filters on both ids: hidapi only runs the free uevent pre-check when ids are
passed (linux/hid.c:962), so an unfiltered walk sends every device straight to
the locked reads. Tests stall via ctypes.PyDLL, which unlike CDLL holds the
GIL — the shape a thread bound cannot cover.

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