bpf: Selftests, add blacklist to test_sockmap

This adds a blacklist to test_sockmap. For example, now we can run
all apply and cork tests except those with timeouts by doing,

 $ ./test_sockmap --whitelist "apply,cork" --blacklist "hang"

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/158939734350.15176.6643981099665208826.stgit@john-Precision-5820-Tower