Fix dbm_gnu test relying on set order.
diff --git a/Lib/test/test_dbm_gnu.py b/Lib/test/test_dbm_gnu.py
index ce96ce4..30a39f7 100755
--- a/Lib/test/test_dbm_gnu.py
+++ b/Lib/test/test_dbm_gnu.py
@@ -53,7 +53,7 @@
         all = set(gdbm.open_flags)
         # Test standard flags (presumably "crwn").
         modes = all - set('fsu')
-        for mode in modes:
+        for mode in sorted(modes):  # put "c" mode first
             self.g = gdbm.open(filename, mode)
             self.g.close()