gh-96853: Restore test coverage for Py_Initialize(Ex) (GH-98212)

* As most of `test_embed` now uses `Py_InitializeFromConfig`, add
  a specific test case to cover `Py_Initialize` (and `Py_InitializeEx`)
* Rename `_testembed` init helper to clarify the API used
* Add a `PyConfig_Clear` call in `Py_InitializeEx` to make
  the code more obviously correct (it already didn't leak as
  none of the dynamically allocated config fields were being
  populated, but it's clearer if the wrappers follow the
  documented API usage guidelines)