TST: Mock model classes in unit tests for high-level functions, write

[NickleDave]

Many unit tests for the "core" functions that correspond to the CLI commands are slow, e.g. for vak.train.frame_classification.train_frame_classification_model. This is because they actually run the trainer on the model class.

• We should if possible speed these up by using mocks
• We should then have separate unit tests for the classes themselves that test they train / eval / predict as expected