zdtm: rollback cwd only if a test case is executed in namespaces

$ bash test/zdtm.sh -i 0 '\(ns/static/env00\|static/pty03\|static/mountpoints\)'
...
==================================== ERROR ====================================
Test: zdtm/live/static/mountpoints, Namespace:
Output file: /root/git/1/criu/test/zdtm/live/static/mountpoints.out
------------------------------------------------------------------------------
00:53:03.267: 26935: FAIL: mountpoints.c:139: Can't umount all the filesystems (errno = 16 (Device or resource busy))
------------------------------------- END -------------------------------------
================================= ERROR OVER =================================

Fixes: 0198590c (zdtm: Factor out starting the test via Makefule)
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -403,7 +403,7 @@ start_test()
 		return 1
 	fi
 
-	[ -z "$ZDTM_ROOT" ] || cd -
+	[ -z "$PIDNS" ] || cd -
 
 	PID=`cat "$TPID"` || return 1
 	if ! ps -p $PID ; then