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.

* 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

* Fix _PyMainInterpreterConfig_Copy():
   copy 'install_signal_handlers' attribute
* Add _PyMainInterpreterConfig_AsDict()
* Add unit tests on the main interpreter configuration
  to test_embed.InitConfigTests
* test.pythoninfo: log also main_config

* Revert "bpo-34589: Add -X coerce_c_locale command line option (GH-9378)"

This reverts commit dbdee0073cf0b88fe541980ace1f650900f455cc.

* Revert "bpo-34589: C locale coercion off by default (GH-9073)"

This reverts commit 7a0791b6992d420dc52536257f2f093851ed7215.

* Revert "bpo-34589: Make _PyCoreConfig.coerce_c_locale private (GH-9371)"

This reverts commit 188ebfa475a6f6aa2d0ea14ca8e1fbe7865b6d27.

Py_Initialize() and Py_Main() cannot enable the C locale coercion
(PEP 538) anymore: it is always disabled. It can now only be enabled
by the Python program ("python3).

test_embed: get_filesystem_encoding() doesn't have to set PYTHONUTF8
nor PYTHONCOERCECLOCALE, these variables are already set in the
parent.

_PyCoreConfig:

* Rename coerce_c_locale to _coerce_c_locale
* Rename coerce_c_locale_warn to _coerce_c_locale_warn

These fields are now private (name prefixed by "_").

_PyCoreConfig_Read() is now responsible to choose the filesystem
encoding and error handler. Using Py_Main(), the encoding is now
chosen even before calling Py_Initialize().

_PyCoreConfig.filesystem_encoding is now the reference, instead of
Py_FileSystemDefaultEncoding, for the Python filesystem encoding.

Changes:

* Add filesystem_encoding and filesystem_errors to _PyCoreConfig
* _PyCoreConfig_Read() now reads the locale encoding for the file
  system encoding.
* PyUnicode_EncodeFSDefault() and PyUnicode_DecodeFSDefaultAndSize()
  now use the interpreter configuration rather than
  Py_FileSystemDefaultEncoding and Py_FileSystemDefaultEncodeErrors
  global configuration variables.
* Add _Py_SetFileSystemEncoding() and _Py_ClearFileSystemEncoding()
  private functions to only modify Py_FileSystemDefaultEncoding and
  Py_FileSystemDefaultEncodeErrors in coreconfig.c.
* _Py_CoerceLegacyLocale() now takes an int rather than
  _PyCoreConfig for the warning.

* Add stdio_encoding and stdio_errors fields to _PyCoreConfig.
* Add unit tests on stdio_encoding and stdio_errors.

Python now gets the locale encoding with C code to initialize the encoding
of standard streams like sys.stdout. Moreover, the encoding is now
initialized to the Python codec name to get a normalized encoding name and
to ensure that the codec is loaded. The change avoids importing
_bootlocale and _locale modules at startup by default.

When the PYTHONIOENCODING environment variable only contains an encoding,
the error handler is now is now set explicitly to "strict".

Rename also get_default_standard_stream_error_handler() to
get_stdio_errors().

Reduce the buffer to format the "cpXXX" string (Windows locale encoding).

If coerce_c_locale is already set (>= 0), use its value: don't
override it.