test/zdtm.sh: use if statements

For longer multi-line statements, let's use traditional

	if ... ; then ... fi

Short form is OK (better?) for one-liners only.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>