test_cext, test_cppext: enable /W4 warnings on Windows (#124253)

Add an explicit cast to (void*) and add Py_UNUSED() to fix some
warnings in extension.c.

Lib/test/test_cppext/setup.py

@@ -22,6 +22,8 @@
 else:
     # MSVC compiler flags
     CPPFLAGS = [
+        # Display warnings level 1 to 4
+        '/W4',
         # Treat all compiler warnings as compiler errors
         '/WX',
     ]