`_testcapi` should be multi-phase

[ZeroIntensity]

_testcapi uses legacy single-phase initialization, which prevents us from being able to use it in subinterpreter tests. Historically, _testcapi was kept as single-phase because we needed a module to test single-phase init, but since we now have _testsinglephase, we can move it to multi-phase init.

This is currently blocking #136004, as _testcapi is needed for one of the tests.

Linked PRs

• gh-139098: Use multiphase initialization in _testcapi #139102