Test Limited API in _testcapi module

[encukou]

The Limited API is currently tested in xxlimited module(s), which isn't a great location since the module serves as a “tutorial” example of how to make a module. There's no room for advanced stuff or edge cases. Testing Limited API together with other C API is complicated, because:

• Limited API needs to be enabled per source file
• Some builds don't support Limited API, so Limited API tests must be skipped on those builds. (Currently this is Py_TRACE_REFS, but that may change.)
• Py_LIMITED_API must be defined before <Python.h> is included.

With #95505 (splitting _testcapi module into several source files), the first complication is solved. I'd like to add helpers for the others, to make testing Limited API easy.