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

[ncoghlan]

• As most of test_embed now uses Py_InitializeFromConfig, add a specific test case to cover Py_Initialize(Ex)
• 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)

• Issue: gh-96853