Missing tests for Influence Line Diagrams

[mvg2002]

While looking through the beam module and the tests for this module I saw that there are no tests for the Influence Line Diagram (I.L.D.) methods. I was looking for this in relation to issue #26601. In test_beam.py not a single I.L.D. method is tested.

The best way forward is probably to first add test for the current functionality of the method, before changing the method to be be able to handle hinges.

As this is for a school project I will tag my supervisors so they stay up-to-date: @Tom-van-Woudenberg @moorepants