selftests/hid: add subprog call test

I got a weird verifier error with a subprog once, so let's have a test
for it.

Link: https://lore.kernel.org/r/20240608-hid_bpf_struct_ops-v3-9-6ac6ade58329@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>