test: add a generic rule to generate directed rings

test/Makefile.dot

-# Time-stamp: <modified the 05/05/2021 (at 13:20) by Erwan Jahier>
+# Time-stamp: <modified the 27/07/2021 (at 10:44) by Erwan Jahier>
 
 # Rules to generate various dot files.
 # The DECO_PATTERN variable should be defined
@@ -32,6 +32,10 @@ ring%.dot:
 	gg ring -n $* -o $@
 	gg-deco $(DECO_PATTERN) $@ -o $@
 
+diring%.dot:
+	gg ring -dir -n $* -o $@
+	gg-deco $(DECO_PATTERN) $@ -o $@
+
 tree%.dot:
 	gg tree -n $* -o $@
 	gg-deco $(DECO_PATTERN) $@ -o $@