fix make clean target

tester/Makefile

@@ -23,9 +23,10 @@ $(OBJ_DIR):
 	mkdir -p $(OBJ_DIR)
 
 clear:
-	rm -r $(OBJ_DIR)/*
+	rm -rf $(OBJ_DIR)/*
 
-clean: clear $(BIN)
+clean: clear
+	rm -f $(BIN)
 
 run: $(BIN)
 	./$(BIN)