Add a couple of tests for fold(). One disabled, because the function is broken.

dkim/tests/test_dkim.py

@@ -0,0 +1,20 @@
+import unittest
+
+import dkim
+
+
+class TestFold(unittest.TestCase):
+
+    def test_short_line(self):
+        self.assertEqual(
+            "foo", dkim.fold("foo"))
+
+    def DISABLED_test_long_line(self):
+        # The function is terribly broken, not passing even this simple
+        # test.
+        self.assertEqual(
+            "foo"*24 + "\r\n foo", dkim.fold("foo" * 25))
+
+
+if __name__ == '__main__':
+    unittest.main()