Create generic 'learnt_attributes' tests

[limlam96]

What?
Many transformers have tests like 'test_learnt_values_not_modified', which currently not abstracted because learnt_attributes have many different names. If we stored linked transformers/learnt_attributes in a test fixture, we could then move these tests into parent test classes and reduce duplication

Why?
Reduce test duplication

How?
Create dict in conftest matching transformers to learnt attributes, and use this fixture to move relevant tests in the generic test classes