pythongh-111644: Enhance support threading_cleanup()

Copy the list of dangling thread to provide a consistent output
between the warning and the "Dangling thread" listing.

Author: vstinner

Lib/test/support/threading_helper.py

@@ -36,16 +36,23 @@ def threading_cleanup(*original_values):
         if not count:
             # Display a warning at the first iteration
             support.environment_altered = True
-            dangling_threads = values[1]
+
+            # Copy the thread list to get a consistent output between the
+            # warning and the "Dangling thread" listing.
+            dangling_threads = list(values[1])
+
             support.print_warning(f"threading_cleanup() failed to cleanup "
                                   f"{values[0] - original_values[0]} threads "
                                   f"(count: {values[0]}, "
                                   f"dangling: {len(dangling_threads)})")
             for thread in dangling_threads:
                 support.print_warning(f"Dangling thread: {thread!r}")
+                breakpoint()
 
-            # Don't hold references to threads
+            # Break any potential reference cycle
             dangling_threads = None
+
+        # Break any potential reference cycle
         values = None
 
         time.sleep(0.01)