selftests/bpf: Fix wq test.

The wq test was missing destroy(skel) part which was causing bpf progs to stay
loaded. That was causing test_progs to complain with
"Failed to unload bpf_testmod.ko from kernel: -11" message, but adding
destroy() wasn't enough, since wq callback may be delayed, so loop on unload of
bpf_testmod if errno is EAGAIN.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Fixes: 8290dba5 ("selftests/bpf: wq: add bpf_wq_start() checks")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>