[TASK] Merge `*-testing-utils` crates into `*-extras` crates

[Serial-ATA]

Overview

By the looks of it, we could just take our testing utils and move them into the extras crates. For example gadget-anvil-testing-utils becomes blueprint_evm_extra::testing.

For gadget-core-testing-utils, they can be moved into blueprint_core::testing.

In all cases, we just have a testing feature, or test-util in case testing sounds like an experimental feature.

This will remove the following crates: gadget-core-testing-utils, gadget-anvil-testing-utils, gadget-tangle-testing-utils, gadget-eigenlayer-testing-utils.