Ensure the NullUser can't withdraw students.

Change-Id: Ia879c5bbe8476d217ca55a560a65d6f6b8839201
Ensure non-authenticated users can't withdraw users either.

Change-Id: I823e51ad2da5d6ac9a1f366c12d287958ff8188b
Add section names in tests.

Change-Id: Ib0b6ab68dc6eb6182f963c0d565ffec8526d1fa2