selftests/bpf: wq: add bpf_wq_init() checks

Allows to test if allocation/free works

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Link: https://lore.kernel.org/r/20240420-bpf_wq-v2-12-6c986a5a741f@kernel.org


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