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>