[Plots] Write basic end-to-end tests for sim plots & arima plots

[trentmc]

Background / motivation

Sim engine plots have now hit a relatively stable macro-state: using plotly, etc.

But we don't have tests for them at the macro level yet.

We do have some macro-level tests for arima plots: in statutil/test/test_arima_dash_plots.py and test_dash_plots_utils.py. However they're not enough either. For example, #1206 was a small bug, yet it prevented arima plots from launching. The existing tests didn't catch it.

TODOs

• Write basic end-to-end tests for sim engine plots (pdr sim_plots), such that after a short sim run, can they plot everything without something crashing. To keep runtime fast, this could be done as part of test_sim_engine.py -> done in Add sim_plots testing with Dash testing (Dash duo) #1228
• Write basic end-to-end tests for arima plots (pdr arima_plots). Test it on the data thattest_seasonal.py::_get_data() uses

Related issues

A good follow-up issue to this one is: #1208 "In sim and pdr bot tests, need to test both classifier and regressor ai models"