selftests/bpf: Add link detach tests for cgroup, netns, and xdp bpf_links

Add bpf_link__detach() testing to selftests for cgroup, netns, and xdp
bpf_links.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200731182830.286260-4-andriin@fb.com