core: ignore errors from cg_create_and_attach() in test mode (#8401)

core: ignore errors from cg_create_and_attach() in test mode (#8401)

Reproducer:

$ meson build && cd build
$ ninja
$ sudo useradd test
$ sudo su test
$ ./systemd --system --test
...
Failed to create /user.slice/user-1000.slice/session-6.scope/init.scope control group: Permission denied
Failed to allocate manager object: Permission denied

Above error message is caused by the fact that user test didn't have its
own session and we tried to set up init.scope already running as user
test in the directory owned by different user.

Let's try to setup cgroup hierarchy, but if that fails return error only
when not running in the test mode.

Fixes #8072