fuzz: Mock CMainSignals in process_message(s) targets #26872

Mocking CMainSignals gives us the ability to control when the CValidationInterface callbacks are called on PeerManager in the process_message(s) targets. It thereby makes these targets more deterministic/reproducable and slightly faster (per eyeball measurements 5%-10% more execs/s on my machine).

Mostly looking for conceptual review on this because there is an argument to be made about fuzzing with the real CMainSignals in combination with TSan, so maybe preserving that behavior in another target could also make sense.

@MarcoFalke curious about your opinion on this (maybe this has been discussed before?)

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For those targets, none of them should be ever hit, practically speaking. It might be better to add a new validationinterface fuzz target

For those targets, none of them should be ever hit, practically speaking.

Ugh, I assumed it's possible because we add those easily spendable coinbase outputs and call SyncWithValidationInterfaceQueue, but yeah looking at the coverage there is no coverage for any of the validation callbacks. The SyncWithValidationInterfaceQueue does slow down the targets, so it's a little annoying to have that when it's practically never needed.

I have been working on targets that more specifically fuzz certain protocol flows (e.g. the version handshake, block/tx relay). These targets mostly submit valid headers/blocks/txs to net_processing, so I guess what I am suggesting here makes more sense for them.