zdtm.py: don't fail tests if pids collide

It seems with the new zdtm.py that pids collide a lot more (perhaps because
everything is in its own pid namespace? not sure). In any case, this is
annoying as it prevents a second run of the tests from actually getting to
the test part.

So, let's try to move the old directory out of the way.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>