test: Test permanent hook failure

This patch adds a test to make sure we handle correctly the case where a
hook exits with a permanent failure.

test/t-10-hooks/config/hooks/post-data.good

@@ -10,5 +10,10 @@ if [ "$RCPT_TO" == "blockme@testserver" ]; then
 	exit 1
 fi
 
+if [ "$RCPT_TO" == "permanent@testserver" ]; then
+	echo "Nos hacemos la permanente"
+	exit 20  # permanent
+fi
+
 echo "X-Post-Data: success"