zdtm: cleanup zdtmtst and zdtmtst.defaultroot cgroups after finishing test

After running criu test, docker on the node becomes unusable, as it is
confused by our leftover cgroups. Surely docker should be fixed to
ignore custom cgroups (https://github.com/moby/moby/issues/37601

), but
we should not leave them after test also.

v2: rmdir the holder only if it exists, remove racy wait and remove
wrongly added cleanup method in class criu
v3: bring back missed semicolon

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>