Add test for partial parent asset case

[alexdewar]

As mentioned in #1351, it would be good to have a test for the case where we are only dispatching part of a parent asset's capacity (i.e. a subset of its commissioned children).

There is a model with this property in #1131, but I'm not sure we'd want it as an example. I wonder if we could get Copilot to find a more minimal example (a small patch example would be good), either based on the model in #1131 or something else.

[alexdewar]

Come to think of it, I wonder if it would work to just tweak the existing simple_divisible model so there are more divisible assets. Then capacity should be exceeded and only a subset will be selected during appraisal.