zdtm: define PAGE_SIZE in zdtmtst.h

This patches fixes compilation on Raspberry Pi and
removes a few defines from tests.

msg.c: In function ‘test_msg’:
msg.c:41:11: error: ‘PAGE_SIZE’ undeclared (first use in this function)

CC: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>