selftests/bpf: add checks for bpf_wq_set_callback()

We assign the callback and set everything up.
The actual tests of these callbacks will be done when bpf_wq_start() is
available.

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Link: https://lore.kernel.org/r/20240420-bpf_wq-v2-14-6c986a5a741f@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>