Add assertion for interval count

Signed-off-by: Thomas Lauf <thomas.lauf@tngtech.com>

test/undo.t

@@ -92,12 +92,12 @@ class TestUndo(TestCase):
         self.t("undo")
 
         j = self.t.export()
+        self.assertEqual(len(j), 2, msg="Expected 2 intervals afterwards, got {}".format(len(j)))
         self.assertOpenInterval(j[1],
                                 expectedStart=one_hour_before_utc,
                                 expectedTags=["bar"],
                                 expectedAnnotation="")
 
-
     def test_undo_cancel(self):
         """Test undo of command 'cancel'"""
         one_hour_before_utc = datetime.now().utcnow() - timedelta(hours=1)