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

Comece revisando Modules/_testcapimodule.c e os módulos existentes _testmultiphase, _testimportmultiple, _testinternalcapi e _testbuffer; em seguida, examine os PRs vinculados listados na issue. O trabalho estará concluído quando os testes da C-API estiverem divididos em módulos coerentes, a cobertura de testes relevante for preservada e a poluição do namespace dos módulos de nível superior for evitada.