Add unit tests for `stat_fit_deviations()`, `stat_fit_residuals()` and `stat_fit_fitted()`

[aphalo]

These stats are currently tested only by examples in the documentation, including the on-line only articles. As the model fit methods supported are open-ended, at least test with all documented cases.

• [ ] Add unit tests for stat_fit_deviations()
• [ ] Add unit tests for stat_fit_residuals()
• [ ] Add unit tests for stat_fit_fitted()