Clean test dir on 'make clean'

Makefile

@@ -64,12 +64,12 @@ clean: subdirs_clean
 	rm -f include/asm include/net include/linux include/bits
 
 subdirs: $(patsubst %, _dir_%, $(DIRS))
-subdirs_clean: $(patsubst %, _dirclean_%, $(DIRS))
+subdirs_clean: $(patsubst %, _dirclean_%, $(DIRS) test)
 
 $(patsubst %, _dir_%, $(DIRS)) : dummy
 	$(MAKE) -C $(patsubst _dir_%, %, $@)
 
-$(patsubst %, _dirclean_%, $(DIRS)) : dummy
+$(patsubst %, _dirclean_%, $(DIRS) test) : dummy
 	$(MAKE) -C $(patsubst _dirclean_%, %, $@) clean
 
 .PHONY: dummy