TODOs in test_gbt_formulation

[jezsadler]

A couple of TODO items in this test file:

1. line 34, # TODO @cog-imperial: fix below?: - unclear what this is referring to
2. line 49, # TODO @cog-imperial: did we remove categorical variables intentionally?, followed by a test for a model with categorical variables. We should make a decision on whether to remove this and either restore or remove the test.

[jezsadler]

Removed these (and the commented test) in my fork, will close on integration.