use a modeline for zdtm.py to set noet

Most python uses spaces, so that's probably what people have their editors
configured to do. Since this file uses tabs, let's be explicit about it.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.py

 #!/usr/bin/env python
+# vim: noet
 import argparse
 import yaml
 import os