circular_linked_list: Add more len() tests

Author: cclauss

data_structures/linked_list/circular_linked_list.py

@@ -37,6 +37,15 @@ def __len__(self) -> int:
         >>> cll.append(1)
         >>> len(cll)
         1
+        >>> cll.preappend(0)
+        >>> len(cll)
+        2
+        >>> cll.delete_front()
+        >>> len(cll)
+        1
+        >>> cll.delete_rear()
+        >>> len(cll)
+        0
         """
         return self.length