test/zdtm: use pr_err() not pr_perror

In places where we are not supposed to print errno, or where
we already print in explicitly, use pr_err().

travis-ci: success for more pr_perror() usage fixes
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>