Modular Unit Tests

[ngupta23]

Break down the auto_ts unit tests into more modular unit tests. IT currently runs like regression tests and not unit tests

• Break down main flow unit tests into separate tests for individual model builds such as the one done for auto_sarimax
• main flow unit tests should only focus on the flow of the automl and not on the output of individual models.

TODO

• [x] arima
• [ ] auto_sarimax
• [x] var
• [ ] Prophet
• [ ] ml
• [ ] Clean auto_ts tests to check for flow related issues only