[ENH] Test `anomaly_detection` estimator `_fit_predict` in general testing

[MatthewMiddlehurst]

Describe the feature or idea you want to propose

This is currently not tested. This should only test estimators when the _fit_predict function is overridden from the base class, no need to test default functionality. Should check that the overridden functions runs as expected, and produced the same output as .fit().predict().

Describe your proposed solution

Write a test and add it the AD checks in the testing module

Describe alternatives you've considered, if relevant

No response

Additional context

No response

[notaryanramani]

Hey, can I take on this issue?

[SebastianSchmidl]

Please wait until #2326 is merged because it introduces a new base class hierarchy for AD, which involves changes to the test architecture as well.