[2.2.x] Refs CVE-2021-31542 -- Skipped mock AWS storage test on Windows.

The validate_file_name() sanitation introduced in
0b79eb3 correctly rejects the example
file name as containing path elements on Windows. This breaks the test
introduced in 914c72b to allow path
components for storages that may allow them.

Test is skipped pending a discussed storage refactoring to support this
use-case.

Backport of a708f39 from main

tests/file_storage/test_generate_filename.py

@@ -1,4 +1,6 @@
 import os
+import sys
+from unittest import skipIf
 
 from django.core.exceptions import SuspiciousFileOperation
 from django.core.files.base import ContentFile
@@ -93,6 +95,7 @@ def upload_to(instance, filename):
             os.path.normpath('some/folder/test_with_space.txt')
         )
 
+    @skipIf(sys.platform == 'win32', 'Path components in filename are not supported after 0b79eb3.')
     def test_filefield_awss3_storage(self):
         """
         Simulate a FileField with an S3 storage which uses keys rather than