Add more tests for investment step

While there are some unit tests in `src/simulation/investment.rs`, there are also many functions without dedicated unit tests. This is particularly true for simple utility functions (e.g. `get_demand_portion_for_market`) with clear inputs and outputs. For more complex functions, we should at least consider how we might test them, e.g. by breaking them up.

Relying on regression tests to detect problems with this part of the code is not a robust approach.