[pylint] Refactor test suite for custom pylint checkers to use files instead of docstrings

[l0lawrence]

Investigate refactoring the test suite of the custom pylint checkers. A few checkers use files to test but the majority just have small little tests. Is there a better way to test the checkers? Right now we pass the node that we want directly into the function to test i.e class_node[0] is there a way to make the test more like the real runs of pylint?

[JessicaBell00]

Have started to work on this :)