selftests/bpf: Test bpf_{get,set}_retval behavior with cgroup/sockopt

The tests checks how different ways of interacting with the helpers
(getting retval, setting EUNATCH, EISCONN, and legacy reject
returning 0 without setting retval), produce different results in
both the setsockopt syscall and the retval returned by the helper.
A few more tests verify the interaction between the retval of the
helper and the retval in getsockopt context.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>