The `_testcapimodule.c` file is getting too large to work with effectively.
This PR lays out a general structure of how tests can be split up, with more splitting to come later if the structure is OK.
Vectorcall tests aren't the biggest issue -- it's just an area I want to work on next, so I'm starting here.
An issue specific to vectorcall tests is that it wasn't clear that e.g. `MethodDescriptor2` is related to testing vectorcall: the `/* Test PEP 590 */` section had an ambiguous end. Separate file should make things like this much clearer.
OTOH, for some pieces it might not be clear where they should be -- I left `meth_fastcall` with tests of the other calling conventions. IMO, even with the ambiguity it's still worth it to split the huge file up.
I'm not sure about the buildsystem changes, hopefully CI will tell me what's wrong.
@vstinner, @markshannon: Do you think this is a good idea?
Automerge-Triggered-By: GH:encukou
Oleg Iarygin
Petr Viktorin