Fix the bug of the temp-files removing

The shell command to remove all the temp-files created had a bug that
try to remove /dev/null. If the ltp is run with root permission the
/dev/null will be removed.

Signed-off-by: jungsoo.son <jungsoo.son@lge.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>