bpo-35233: Rewrite test_embed.InitConfigTests (GH-10524)

* Fix _PyCoreConfig_SetGlobalConfig(): set also Py_FrozenFlag
* Fix _PyCoreConfig_AsDict(): export also xoptions
* Add _Py_GetGlobalVariablesAsDict() and _testcapi.get_global_config()
* test.pythoninfo: dump also global configuration variables
* _testembed now serializes global, core and main configurations
  using JSON to reuse _Py_GetGlobalVariablesAsDict(),
  _PyCoreConfig_AsDict() and _PyMainInterpreterConfig_AsDict(),
  rather than duplicating code.
* test_embed.InitConfigTests now test much more configuration
  variables