Test directory structure for models doesn't match corresponding implementation files

[ebsmothers]

Right now we have e.g. tests/torchtune/models/test_llama3.py and tests/torchtune/models/test_lora_llama2.py test files for our models. This is not in line with our implementation files, which have directories torchtune/models/llama2, torchtune/models/mistral, etc. We should refactor the tests/torchtune/models directory to better line up with the directory structure of torchtune/models to improve discoverability of our test files.