gh-93649: Split vectorcall testing from _testcapimodule.c by encukou · Pull Request #94549 · python/cpython

The _testcapimodule.c file is getting too large to work with effectively.
Vectorcall tests aren't the biggest issue -- it's just an area I want to work
on next, so I started there.
It does make it clear that MethodDescriptor2 is related to testing vectorcall,
which wasn't clear before (the /* Test PEP 590 */ section had an ambiguous end).

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.

Co-authored-by: Victor Stinner <vstinner@python.org>