Consistent naming and grouping scheme

[ladc]

Devise a logical naming scheme and hierarchy / directory structure for the tests.

[ladc]

This appears to be a tough one, so I've created a wiki page with a first and very incomplete proposal for organizing the tests. The declarative approach suggested by @agentzh hopefully allows us to specify different tiers (e.g. slow/fast tests) by "tagging" the various tests (possibly within a same file) without having to create a very deeply nested (directory) structure.

[Wiladams]

I suggest we go ahead and create the specified structure, and start moving tests into those directories. I don't think we need to wait on any particular framework to do this work. We can always rearrange later to match any future framework requirements, but we're stalled waiting on frameworks right now.

[ladc]

The proposed structure is a first suggestion and is definitely incomplete; feedback is more than welcome. I'll start reorganizing tests into this structure now anyway. I guess it'll become clear soon enough where it's lacking.