test/zdtm.sh: require bash

There are a lot of bash-only statements in this script, so let's
ask for /bin/bash explicitly to avoid weird errors when running
on e.g. Debian which have dash installed by default.

Alternatively, we can rewrite this to be pure shell, but it's
too much work to do.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

-#!/bin/sh
+#!/bin/bash
 
 ZP="zdtm/live"