test_embed.InitConfigTests tests more configuration variables.

Changes:

* InitConfigTests tests more core configuration variables:

  * base_exec_prefix
  * base_prefix
  * exec_prefix
  * home
  * legacy_windows_fs_encoding
  * legacy_windows_stdio
  * module_search_path_env
  * prefix

* "_testembed init_from_config" tests more variables:

  * argv
  * warnoptions
  * xoptions

* InitConfigTests: add check_global_config(), check_core_config() and
  check_main_config() subfunctions to cleanup the code. Move also
  constants at the class level (ex: COPY_MAIN_CONFIG).
* Fix _PyCoreConfig_AsDict(): don't set stdio_encoding twice
* Use more macros in _PyCoreConfig_AsDict() and
  _PyMainInterpreterConfig_AsDict() to reduce code duplication.
* Other minor cleanups.