zdtm.py: call clean_tests_root() from one process only

An atexit hook is executed for forked processes too,
clean_tests_root() has to be called only once.

v2: fix flak8 warnings
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>