selftests/bpf: test_sockmap, join cgroup in selftest mode

In case of selftest mode, temporary cgroup environment is created but
cgroup is not joined. It causes test failures. Fixed by joining the
cgroup

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>