Aside from the normal "is it valid" test, does anyone know of a module that holds all of the possible DNN controls that you can insert into a site to verify that all the controls will render appropriately for the skin and container you plan to use?

It may not catch everything, but it seems having something like this would at least catch most of the problems one is likely to find as you start using a skin.

It does in a way. You create the skin as HTML file and parse it with DNN for you tests (which is why I never do this, to much hassle)
So that way you can test the skin in DNN as a normal ascx skin.
Then if you package the skin as an HTML skin.

Now you can install the html skin on another DNN installation and as non existent skin objects are not replaced by the parser, the skin does work normally and does not crash on a single not installed skin object, like an ascx skin does.

This is the reason many skin vendors stil create HTML skins, while most custom skin developers create ascx skins.

A module as you describe does not exist AFAIK.

BTW, maybe you could try to sound less cynical, that results in a higher probability you get your questions answered...