selftests/bpf: Add link pinning selftests

Add selftests validating link pinning/unpinning and associated BPF link
(attachment) lifetime.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200303043159.323675-4-andriin@fb.com