The reason I didn't write a test is that ghostscript actually doesn't error when rendering the "bad" ps files, just gives a blank page, and as usual I didn't want to add a baseline image. I guess one could use the usual trick of rendering the thing, "manually" (i.e. by calling ghostscript in the test) converting to png, and checking that the page isn't all white...

I went through a very similar thought process. Would it be reliable to render the example@QuLogic came up with verify that the mediabox is correct?

I guess "yes", but it's really a super indirect test, it wouldn't be obvious at all why these values are the correct ones.