infra: split model unit tests by Model, FrameworkModel, and ModelPackage

[laurenyu]

Issue #, if available:

Description of changes:
I was having a hard time disambiguating which model/deploy tests were testing FrameworkModel functionality versus just Model functionality because our unit tests just indiscriminately use DummyFrameworkModel for everything. I decided to split up the file into three files first, and then after this I'll work on converting some more DummyFrameworkModel tests into Model tests (eyeing some of the deploy() tests).

In this PR, I didn't change any code within the tests - everything's just a copy/paste.

I also created a new folder tests/unit/sagemaker/model to follow the directory structure started by AutoML and Model Monitoring to hopefully help with organization.

Testing done:
pytest tests/unit/sagemaker/model/

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