Add more unit tests for models that could use them

[emlys]

Some of our older models' test suites only test the final model results, which is effective at catching bugs, but not very helpful in identifying where a bug originated. It would be great if all models had least some unit tests that check intermediate results. Not every single function needs a unit test - some are pretty trivial.

The task:
Identify functions that contain significant logic and/or occur at a critical point in the model. Write unit tests for these functions and/or assert that intermediate results are produced correctly.

Some suggestions to get started with:

• Annual water yield: unit test for fractp_op
• Forest carbon: unit test for _calculate_tropical_forest_edge_carbon_map

This is complete when all invest models have at least a couple of unit tests that will help us identify issues in the intermediate steps of the models.