selftests/hid: extract the utility part of hid_bpf.c into its own header

When adding new tests programs, we need the same mechanics to create
new virtual devices, and read from their matching hidraw node.

Extract the common part into its own header so we can easily add new
tests C-files.

Link: https://patch.msgid.link/[email protected]
Signed-off-by: Benjamin Tissoires <[email protected]>

Author: Benjamin Tissoires