Split Up _testsinglephase.c and _testmultiphase.c

(low priority)

Modules/_testsinglephase.c and Modules/_testmultiphase.c both contain the implementations of multiple modules. We should consider splitting up both files, so each contained module has its own file.

Here are several reasons why this may be worth doing:

• testing with a module hidden in another's .so file is awkward and makes those tests harder to understand
• it's easier to accidentally leak C-module global data between modules when they are implemented in the same file
• when adding a new module, it's trickier (less familiar) to implement it in an existing file than in a new file [^1]

[^1]: However, adding it to an existing file is also simpler in how adding a new file involves tweaking the build. See the devguide.

Furthermore, both files were added to test specific import functionality. As far as I can tell[^2], that never included any need for the multiple-modules-in-one-file approach. However, currently they are the only place where we're testing the feature, regardless of how unintentionally or superficially. gh-124978 targets fixing that lack of explicit testing, including moving away from _testsinglephase.c and _testmultiphase.c (e.g. using the dedicated Modules/_testimportmultiple.c).

[^2]: I personally added Modules/_testsinglephase.c and @encukou added _testmultiphase.c.

All that said, splitting up the files is a fairly low priority task. The status quo isn't ideal but it isn't that bad either.

Here's how I see us taking care of this:

1. make sure multiple-modules-in-one-file is tested via the dedicated Modules/_testimportmultiple.c [^2] (see gh-124978)
2. create the Modules/_testsinglephase directory
3. move each module from _testsinglephase.c to its own file (see the devguide)
4. update the relevant tests
5. delete Modules/_testsinglephase.c
6. repeat steps 2-5 for Modules/_testmultiphase.c

Comienza revisando Modules/_testsinglephase.c y Modules/_testmultiphase.c, las pruebas relevantes y gh-124978; después, sigue el flujo de trabajo de módulos de extensión de devguide. Se considera terminado cuando el comportamiento de múltiples módulos está cubierto explícitamente, cada módulo incluido tiene su propio archivo, las pruebas relevantes se han actualizado y se han eliminado ambos archivos originales.