Create a TestUtil submodule

[SamuelBrand1]

          This is great. I think we might want a new issue to make a testutils to 1. Make some of the repeated code into test utilities and 2. make plots of the output. We can then generate these plots at test time and load them as an environment object in CI (for visual checking). We could even set up posting a comment with the model output if we think that is useful.

Originally posted by @seabbs in #371 (review)

This has also been floated as an idea when fixing stochastic CI failures, but I think it deserves its own issue.