bpf: add selftest for bpf's (set|get)_sockopt for SAVE_SYN

adding selftest for feature, introduced in commit 9452048c79404 ("bpf:
add TCP_SAVE_SYN/TCP_SAVED_SYN options for bpf_(set|get)sockopt").

Signed-off-by: Nikita V. Shirokov <tehnerd@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>