Fix and test `PyABIInfo_VAR`

# 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-142230
* gh-142833