Fix pickling tests with spurious xfails in convolution - #18453

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Why "spurious" in the title? Can you please provide more context? Thanks!

This goes back to #1947 by blame, from which I quote: "Some of the failures are currently xfails by design (e.g. coordinates.DynamicTransform, which can have a function as an attribute)."

I don't know enough to say whether these fails should have stopped happening in the intervening decade.

If pickling Gaussian1DKernel and Gaussian2DKernel instances succeeds then why should the tests xfail?