test_util: tolerate os.statvfs missing on windows

The code in util.py can tolerate a missing os.statvfs, but the code which
tests that code's ability to tolerate a missing os.statvfs was itself unable
to tolerate a missing os.statvfs. Sigh.

src/wormhole/test/test_util.py

@@ -51,5 +51,10 @@ class Space(unittest.TestCase):
         # anything more specific about the return value
 
     def test_no_statvfs(self):
-        with mock.patch("os.statvfs", side_effect=AttributeError()):
-            self.assertEqual(util.estimate_free_space("."), None)
+        # this mock.patch fails on windows, which is sad because windows is
+        # the one platform that the code under test was supposed to help with
+        try:
+            with mock.patch("os.statvfs", side_effect=AttributeError()):
+                self.assertEqual(util.estimate_free_space("."), None)
+        except AttributeError: # raised by mock.get_original()
+            pass