tests: remember to clean everything on make clean

If we forget to clean some of this stuff, we can get into strange states
where zdtm.py thinks it has constructed the root, but really hasn't, and
can't construct the root because it partially exists.

v2: kill all the extra debugging crap from v1

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>