Add Mistral Model to `test_decoders.py` and generate `expectations`

[rzbhatti]

mistralai/Mistral-7B-Instruct-v0.3 model is enabled with FSM.
foundation-model-stack/foundation-model-stack#395

We need to add tests for mistralai/Mistral-7B-Instruct-v0.3 model over the AIU.