tests: fix lint warnings for zdtm.py

flake8 was updated recently and now it shows a few new warnings:

[root@fc24 criu]# make lint
flake8 --config=scripts/flake8.cfg test/zdtm.py
test/zdtm.py:181:4: E722 do not use bare except'
test/zdtm.py:304:2: E722 do not use bare except'
test/zdtm.py:325:3: E722 do not use bare except'
test/zdtm.py:445:3: E722 do not use bare except'
test/zdtm.py:573:4: E722 do not use bare except'
test/zdtm.py:1369:2: E722 do not use bare except'
test/zdtm.py:1385:3: E722 do not use bare except'
test/zdtm.py:1396:2: E722 do not use bare except'
test/zdtm.py:1420:3: E722 do not use bare except'
test/zdtm.py:1820:2: E741 ambiguous variable name 'l'
make: *** [Makefile:369: lint] Error 1

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