zdtm: check /proc/sys/kernel/taint and fail if it is non-zero

We don't want to run tests, if a kernel has a problem. And we want
to know when the taint flag is changed.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>