lxc-test-apparmor-mount / lxc-test-unpriv in ubuntu_lxc failed with Permission denied with operation under /sys/fs/cgroup/ on Impish

This is not a regression, the ubuntu_lxc test used to get killed with timeout on api_test.py test (bug 1952036) and result in an `I` state on our test result matrix.

After we have changed how we nuke our thread (https://git.launchpad.net/~canonical-kernel-team/+git/autotest/commit/?id=bfd31f0055b881f3cb31f286fe33de2fa39aeef2) the ubuntu_lxc can be reviewed on the matrix now.

Test log - lxc-test-apparmor-mount:
 Running '/tmp/lxc-pkg-ubuntu/src/tests/lxc-test-apparmor-mount'
 /usr/sbin/deluser: The user `lxcunpriv' does not exist.
 /tmp/lxc-pkg-ubuntu/src/tests/lxc-test-apparmor-mount: 152: cannot create /sys/fs/cgroup/-.mount/lxctest/tasks: Permission denied
 lxc-destroy: tmp.Be06aQwmft: tools/lxc_destroy.c: main: 242 Container is not defined
 umount: /sys/kernel/security/apparmor/features/mount: not mounted.
 sed: can't read /run/lxc/nics: No such file or directory

Test log - lxc-test-unpriv:
 Running '/tmp/lxc-pkg-ubuntu/src/tests/lxc-test-unpriv'
 /tmp/lxc-pkg-ubuntu/src/tests/lxc-test-unpriv: line 163: /sys/fs/cgroup/-.mount/lxctest/tasks: Permission denied
 cat: /tmp/tmp.wSJrdfCb6N: No such file or directory