gh-127906: Backport test_cppext changes from main

Author: vstinner

Lib/test/test_cppext/__init__.py

@@ -76,6 +76,8 @@ def run_cmd(operation, cmd):
         cmd = [python_exe, '-X', 'dev',
                '-m', 'pip', 'install', '--no-build-isolation',
                os.path.abspath(pkg_dir)]
+        if support.verbose:
+            cmd.append('-v')
         run_cmd('Install', cmd)
 
         # Do a reference run. Until we test that running python

Lib/test/test_cppext/setup.py

@@ -10,6 +10,7 @@
 
 
 SOURCE = 'extension.cpp'
+
 if not support.MS_WINDOWS:
     # C++ compiler flags for GCC and clang
     CPPFLAGS = [
@@ -19,8 +20,13 @@
         '-Werror',
     ]
 else:
-    # Don't pass any compiler flag to MSVC
-    CPPFLAGS = []
+    # MSVC compiler flags
+    CPPFLAGS = [
+        # Display warnings level 1 to 4
+        '/W4',
+        # Treat all compiler warnings as compiler errors
+        '/WX',
+    ]
 
 
 def main():