bpo-33042: Fix pre-initialization sys module configuration (GH-6157)

bpo-33042: Fix pre-initialization sys module configuration (GH-6157)

- new test case for pre-initialization of sys.warnoptions and sys._xoptions
- restored ability to call these APIs prior to Py_Initialize
- updated the docs for the affected APIs to make it clear they can be
  called before Py_Initialize
- also enhanced the existing embedding test cases
  to check for expected settings in the sys module