[ENH] forecasting coverage

[TonyBagnall]

coverage for forecasting, will aim to keep it at 100%, leaving ETS until after the refactor

[aeon-actions-bot]

Thank you for contributing to aeon

I have added the following labels to this PR based on the title: [ enhancement ].
I would have added the following labels to this PR based on the changes made: [ forecasting ], however some package labels are already present.

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[alexbanwell1]

Looks like good additional coverage, looks good

[alexbanwell1]

Looks good, for increase in coverage