Adding tests for calibration module

[PrasannaPal21]

Currently test_calibration.py doesnt have any tests in it.
the tests are needed for bottle_matchup, robust_linear_fit, model_metrics, model_figs.

I'd like to add tests for these. I am thinking of using synthetic data so the expected outputs are predictable. The existing test data in test/data/ could also work for integration-style tests.

Is there anything I should keep in mind before starting??