vircgrouptest: call virCgroupNewSelf instead virCgroupDetectMounts

This will be required once cgroup v2 is introduced. The cgroup
detection is not simple and we will have multiple backends so we
should not just jump into the middle of the detection code.

In order to use virCgroupNewSelf we need to create all the remaining
data files:

- {name}.cgroups represents /proc/cgroups, it is a list of cgroup
controllers compiled into kernel

- {name}.self.cgroup represents /proc/self/cgroup, it describes
cgroups to which the process belongs

For "no-cgroups" we need to modify the expected behavior because
virCgroupNewSelf() will fail if there are no controllers available.

Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>