fixed tesorflow.keras.activations.get

[ZJay07]

PR Description

flatten_and_to_np needed backend variable, so passed it down from the test function, the lambda function isn't filtering out some activations that can't be tested but in keras.activations, instead I filter out the functions that can be tested (got the list from ivy/functional/frontends/tensorflow/keras/activations.py)

Related Issue

Close #28636
Close #28635
Close #28634
Close #28633
Close #28632

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• Did you add a function?
• Did you add the tests?
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[Ishticode]

looks good to me. Thanks for the effort in this fix @ZJay07
Much appreciated