Fix the following issues in test-cgroup:
1, commit 65be7e0 ("pid1: do not reset subtree_control on
already-existing units with delegation") changed the return value
of cg_create () as follows:
"Returns 0 if the group already existed, 1 on success, negative
otherwise."
So we need to modify the test cases related to cg_create ().

2, commit efdb023 ("core: unified cgroup hierarchy support")
changed cg_delete () to cg_rmdir (), so the test cases also need
to be adjusted a bit.

3. There is no cleanup of "test-a". If we execute test-cgroup
multiple times, we will encounter an error.

Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>

Those cryptic one letter variable names, yuck!

systemd#14901 (comment).

Nowadays with delegation to the user instance, we can make this work as non-root
easily. If we still get access denied, just skip the test.