[capi-sig] Splitting up _testcapimodule.c

Hello list, The _testcapimodule.c file is growing very big, to the point where I started to dread touching it. When adding a test I'm never sure if I'm missing a group of similar ones that it should be grouped with. It's difficult to get a sense of how tested some area of the API is. Code for a single topic is frequently split across several places (functions, type definitions, slot entries...), making me reach for my editor's bookmark feature more than I'd like. Do you feel the same?

I opened an issue, and a PR where I suggest how to split things up. Preferably, please keep the discussion there.