Some tests did not pass on repeated calls (regrtest -R::)
Perform additional cleanup, mostly deleting from sys.modules, or clearing the warnings registry.
diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
index fb6aabb..9fca080 100644
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -391,6 +391,8 @@
 
 
 def test_main():
+    py_warnings.onceregistry.clear()
+    c_warnings.onceregistry.clear()
     test_support.run_unittest(CFilterTests,
                                 PyFilterTests,
                                 CWarnTests,