selftests/bpf: add sockopt tests for TCP_BPF_SOCK_OPS_CB_FLAGS

Add tests to set TCP sockopt TCP_BPF_SOCK_OPS_CB_FLAGS via
bpf_setsockopt() and use a cgroup/getsockopt program to retrieve
the value to verify it was set.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/20240808150558.1035626-3-alan.maguire@oracle.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>