selftests/bpf: Add selftest for bpf_task_under_cgroup() in sleepable prog

The result is as follows:

  $ tools/testing/selftests/bpf/test_progs --name=task_under_cgroup
  #237     task_under_cgroup:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Without the previous patch, there will be RCU warnings in dmesg when
CONFIG_PROVE_RCU is enabled. While with the previous patch, there will
be no warnings.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20231007135945.4306-2-laoar.shao@gmail.com