gh-108303: Add Lib/test/test_cppext/ sub-directory (#108325)

* Move test_cppext to its own directory
* Rename setup_testcppext.py to setup.py
* Rename _testcppext.cpp to extension.cpp
* The source (extension.cpp) is now also copied by the test.

Lib/test/setup_testcppext.py

@@ -1,43 +0,0 @@
-# gh-91321: Build a basic C++ test extension to check that the Python C API is
-# compatible with C++ and does not emit C++ compiler warnings.
-import os
-import sys
-from test import support
-
-from setuptools import setup, Extension
-
-
-MS_WINDOWS = (sys.platform == 'win32')
-
-
-SOURCE = support.findfile('_testcppext.cpp')
-if not MS_WINDOWS:
-    # C++ compiler flags for GCC and clang
-    CPPFLAGS = [
-        # gh-91321: The purpose of _testcppext extension is to check that building
-        # a C++ extension using the Python C API does not emit C++ compiler
-        # warnings
-        '-Werror',
-    ]
-else:
-    # Don't pass any compiler flag to MSVC
-    CPPFLAGS = []
-
-
-def main():
-    cppflags = list(CPPFLAGS)
-    std = os.environ["CPYTHON_TEST_CPP_STD"]
-    name = os.environ["CPYTHON_TEST_EXT_NAME"]
-
-    cppflags = [*CPPFLAGS, f'-std={std}']
-
-    cpp_ext = Extension(
-        name,
-        sources=[SOURCE],
-        language='c++',
-        extra_compile_args=cppflags)
-    setup(name='internal' + name, version='0.0', ext_modules=[cpp_ext])
-
-
-if __name__ == "__main__":
-    main()