[3.7] bpo-35233: Rewrite test_embed.InitConfigTests (GH-10524) (GH-10529)

[3.7] bpo-35233: Rewrite test_embed.InitConfigTests (GH-10524) (GH-10529)

* Add C functions:

  * _Py_GetGlobalVariablesAsDict()
  * _PyCoreConfig_AsDict()
  * _PyMainInterpreterConfig_AsDict()

* Add new _testcapi methods:

  * get_global_config()
  * get_core_config()
  * get_main_config()

* test.pythoninfo: get global, core and main configuration
* _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