test: include any 'stderr' output in the log file.

Errors from mdadm go to 'stderr', so if there is an array,
copy those to the log file.

Signed-off-by: NeilBrown <neilb@suse.de>

test

@@ -240,6 +240,7 @@ do_test() {
       _fail=0
     else
       log=log
+      cat $targetdir/stderr >> $targetdir/log
       if [ $exitonerror == 0 ]; then
 	  log=log-`basename $_script`
 	  mv $targetdir/log $targetdir/$log