[m-dev.] trivial diff: tests/valid/{Mmakefile,runtests} and --use-subdirs

[Zoltan Somogyi]

> +printobjs:
> +	@echo $(OBJS)
> +
>  clean:
>  	rm -f *.c *.o *.err *.d *.h

I think the clean rules in the various directories also need updating.

Zoltan.