selftests/bpf: Allow auto port binding for cgroup connect

Allow auto port binding for cgroup connect test to avoid binding conflict.

Result:
./test_progs -a cgroup_v1v2
59      cgroup_v1v2:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20250227142646.59711-2-jiayuan.chen@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>