Implement dnadna predict --test option

In #68 (closed) @jcury mentioned the possibility of being able to run

dnadna predict --test <path-to-model>

to run the model on the test subset of the dataset it was trained on (if a test set was configured during preprocessing).

This also ties into discussions on #14 (closed) and !100 (merged) regarding how to make the test set actually usable in an easy way.

Currently the .pth files for the models do not contain all the necessary information to load the test set of the associated dataset it was trained on (particularly since !94 (merged))

However, dnadna predict does support passing both a model, and a dataset config file. The dataset config file should set the scenario_params_path to the pre-processed scenario params table that contains all the dataset splits.

This currently not quite so easy to do as one would like, so this this might require more rethinking about how we manage dataset configurations and/or what goes into the .pth file.

(If the test set is small enough, there could even be an option to include it directly in the .pth...)