[epubcheck] Test file directory structure proposal

Hello epubchckers,

I've put together a proposal for this action item with Brady's blessing
(and one noted area of disagreement).

The document can be found here
<https://docs.google.com/document/d/1v7-aSgdA-6dd-5zOTOqwAJGlh2Aa4hYFX1TcCdDesYo/edit#heading=h.ymvhjyrr5mdi>
.

Everybody with the link has comment access, and if specific folks would
like to edit, just let me know.

Of course, conversation can happen on this thread too.

Have a good weekend (which I'm starting early just about now).

Best,
   Garth