Split up _testcapimodule.c

Modules/_testcapimodule.c is a nearly-8000-line behemoth with no clear structure or organization. It is getting hard to maintain.

It also doesn't work well with testing (a) feature macros that affect Python.h and (b) module initialization, so we have additional C-API testing modules: _testmultiphase, _testimportmultiple, _testinternalcapi.
_testbuffer is already split out, but there are many other aspects of the API that would use a similar dedicated test suite.

We should split and combine these, ideally without polluting the namespace of top-level modules.

• PR: gh-94549

• PR: gh-94695

• PR: gh-94702

• PR: gh-95386

• PR: gh-95793

• PR: gh-95819

• PR: gh-99346

• PR: gh-99494

• PR: gh-99517

• PR: gh-99532

• PR: gh-99549

• PR: gh-99551

• PR: gh-102173

• PR: gh-104403

• PR: gh-129400

• PR: gh-129512

• PR: gh-129516

• PR: gh-129521

• PR: gh-129544

Inizia esaminando Modules/_testcapimodule.c e i moduli esistenti _testmultiphase, _testimportmultiple, _testinternalcapi e _testbuffer, quindi esamina le PR collegate elencate nell’issue. Il lavoro è completato quando i test della C-API sono suddivisi in moduli coerenti, la copertura di test rilevante è preservata e si evita l’inquinamento del namespace dei moduli di primo livello.