The new test exercises the most important variants for single-phase init extension modules. We also add some explanation about those variants to import.c.
https://github.com/python/cpython/issues/101758
This makes it more clear that a given test is definitely testing against a single-phase init (legacy) extension module. The new module is a companion to _testmultiphase.
https://github.com/python/cpython/issues/98627