Move BFT insecure code to BFTune repo

Issue To Be Solved

The code inside https://github.com/onflow/flow-go/tree/master/insecure is part of the BFT testing framework, BFTune. This code was meant to stand in a separate BFTune repository. The insecure code introduces vulnerabilities in Flow in order to make it more testable for different attack scenarios. The original plan was to move this code out to a separate repository and then consume it within flow-go. That way, flow-go doesn't have vulnerabilities baked into the code that were only meant for testing environments.

Suggest A Solution

@thep2p and @gomisha volunteer to:

• move this code to the BFTune repository and make sure all Flow integration tests are still passing
• move all BFT integration tests into the BFTune repository

References

BFT Testing Framework for Flow Blockchain - Conference Paper
BFTune: BFT Testing Framework for Flow Blockchain - Blockchain '23 Congress - Presentation on YouTube

cc @peterargue @vishalchangrani @Kay-Zee @AlexHentschel @dete

Start by reviewing flow-go's insecure/ directory and integration/tests/bft, then compare the BFTune repository structure and integration points. Confirm how the Flow integration tests consume this code. Done means the insecure code and BFT integration tests reside in BFTune and all Flow integration tests still pass.