zdtm: fix warnings from flake8

test/zdtm.py:1535:30: E711 comparison to None should be 'if cond is not None:'
test/zdtm.py:1538:31: E711 comparison to None should be 'if cond is not None:'
test/zdtm.py:1601:31: E711 comparison to None should be 'if cond is not None:'

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