gh-95991: Add some infrastructure for testing Limited API in _testcapi (GH-95992)

gh-95991: Add some infrastructure for testing Limited API in _testcapi (GH-95992)

- Limited API needs to be enabled per source file
- Some builds don't support Limited API, so Limited API tests must be skipped on those builds
  (currently this is `Py_TRACE_REFS`, but that may change.)
- `Py_LIMITED_API` must be defined before `<Python.h>` is included.

This puts the hoop-jumping in `testcapi/parts.h`, so individual
test files can be relatively simple. (Currently that's only
`vectorcall_limited.c`, imagine more.)