Remove test.test_support.guard_warnings_filter.
test.test_support.catch_warning is more full-featured and provides the same
functionality.

Since guard_warnings_filter was added in 2.6 there is no
backwards-compatibility issues.
diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index 77bccf6..29fe42a 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -178,7 +178,7 @@
 
     def test_bigrand(self):
         # Verify warnings are raised when randrange is too large for random()
-        with test_support.guard_warnings_filter():
+        with test_support.catch_warning():
             warnings.filterwarnings("error", "Underlying random")
             self.assertRaises(UserWarning, self.gen.randrange, 2**60)