bug#30198: 26.0.91; Fix emacs-module-tests to work with out of tree build directory

[Eli Zaretskii]

> From: Andy Moreton <andrewjmoreton@gmail.com>
> Date: Fri, 26 Jan 2018 13:58:04 +0000
> 
> This also applies cleanly (and works) on emacs-26, if you feel that
> it is not too disruptive for the release branch.

I thought about that, but eventually decided it wasn't worth the risk,
as the problem is not urgent to fix.

Thanks.