Revert "test-exec: do not skip all the tests"

This reverts commit 68e68ca8106e7cd874682ae425843b48579c6539. We *need*
root access to create cgroups. The only exception is if it is run from
within a cgroup with "Delegate=yes". However, this is not always true and
we really shouldn't rely on this.

If your terminal runs from within a systemd --user instance, you're fine.
Everyone else is not (like running from ssh, VTs, and so on..).