pythongh-71052: fix test_concurrent_futures wasi regression. (python#115923)

Fix the WASI test_concurrent_futures regression from python#115917.

Author: gpshead

Lib/test/test_concurrent_futures/__init__.py

@@ -1,7 +1,11 @@
 import os.path
 import unittest
 from test import support
-from test.support import import_helper
+from test.support import threading_helper
+
+
+# Adjust if we ever have a platform with processes but not threads.
+threading_helper.requires_working_threading(module=True)
 
 
 if support.check_sanitizer(address=True, memory=True):