selftests/bpf: add bpf_wq tests

We simply try in all supported map types if we can store/load a bpf_wq.

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Link: https://lore.kernel.org/r/20240420-bpf_wq-v2-10-6c986a5a741f@kernel.org


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