Make layers work with class and module fixtures

[warsaw]

I use layers extensively in Mailman 3 and they generally work great, except for this caveat. Since I haven't seen an issue on this, I want to at least capture the wishlist that module and class fixtures (e.g. setUpModule(), setUpClass() and the associated tear downs) should be made to work with layers. I had a need for that in a recent attempt at refactoring some test code in Mailman 3, since there wasn't any other way to make what I needed to work.

If this is within the project's plans, it might be something I'm willing to work on, though I have no idea how difficult it would be.