test__join_cgroup() combines the following operations that usually
go hand in hand and returns cgroup fd:

  * setup cgroup environment (make sure cgroupfs is mounted)
  * mkdir cgroup
  * join cgroup

It also marks a test as a "cgroup cleanup needed" and removes cgroup
state after the test is done.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>