gh-101659: Clean Up the General Import Tests for Subinterpreters by ericsnowcurrently · Pull Request #103151 · python/cpython

pythongh-103151)

This involves 3 changes: some general cleanup, checks to match the kind of module, and switch from testing against sys to _imp.

This is a precursor to pythongh-103150, though the changes are meant to stand on their own.