[ENH] Tests for feature based classifiers

[TonyBagnall]

adds some coverage for feature based classifiers. After core dev discussion, standardised model for public self.estimator_ for estimator set in fit

[aeon-actions-bot]

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].
I would have added the following labels to this PR based on the changes made: [ $\color{#BCAE15}{\textsf{classification}}$ ], however some package labels are already present.

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