Tests: Investigate `OrderedModelManager` warning for Sponsor models

[JacobCoffee]

Investigate separately these non-failing warnings

WARNINGS:
SponsorshipBenefit: (ordered_model.W003) OrderedModelBase subclass has a ModelManager that does not inherit from OrderedModelManager. This is not ideal but will work.
SponsorshipPackage: (ordered_model.W003) OrderedModelBase subclass has a ModelManager that does not inherit from OrderedModelManager. This is not ideal but will work.

Originally posted by @JacobCoffee in #2741 (comment)