Fix and test `PyABIInfo_VAR`

[philthompson10]

Bug report

Bug description:

PyABIInfo_VAR in modsupport.h generates incorrect code because _PyABIInfo_DEFAULT has been defined as a function. The parentheses should be removed.

CPython versions tested on:

3.15

Operating systems tested on:

macOS

Linked PRs

• gh-142225: Fix PyABIInfo_VAR macro #142230

[encukou]

Thanks!
Keeping the issue open, I'll add tests later.