Add test.test_support.guard_warnings_filter .  This function returns a context
manager that protects warnings.filter from being modified once the context is
exited.
diff --git a/Misc/NEWS b/Misc/NEWS
index 667ccaf..4d9c899 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -302,6 +302,10 @@
 Tests
 -----
 
+- Added guard_warnings_filter to test.test_support.  It returns a context
+  manager that protects the 'warnings' module's filter from being mutated
+  once the context has been exited.
+
 - Added some tests for modulefinder.
 
 - Converted test_imp to use unittest.