Remove test directories in the 'clean' target

shell/Makefile

@@ -13,6 +13,7 @@ main: $(OBJS)
 
 clean:
 	rm -f *.o sh core out
+	rm -r test-dir-*
 
 shell-test: shell-test.c
 	$(CC) -D_DEFAULT_SOURCE -Wall $< -o $@