bpf: Selftests, add whitelist option to test_sockmap

Allow running specific tests with a comma deliminated whitelist. For example
to run all apply and cork tests.

 $ ./test_sockmap --whitelist="cork,apply"

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/158939732464.15176.1959113294944564542.stgit@john-Precision-5820-Tower