zdtm.py: use /usr/bin/env for interpreter

On debian/ubuntu systems, we don't have a /bin/env by default; on rhel
systems (my limited testing seems to indcate that) /usr/bin/env is a
symlink to /bin/env, so let's just use that so it works everywhere.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.py

-#!/bin/env python
+#!/usr/bin/env python
 import argparse
 import yaml
 import os