selftests/bpf: Add bpf link iter test

Add a simple test for bpf link iterator

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Link: https://lore.kernel.org/r/20220510155233.9815-5-9erthalion6@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>