I can't claim to know what all the changes entail, but my understanding is that knots primarily changes transaction relaying policies. That logic could be completely jacked and your node would probably still run fine. The tests for that specifically would amount to, "when I hit the node with this set of transactions, are the ones I expect to be filtered out being filtered out?"
I agree that there ought to be more eyes on the diff between core and knots to ensure that nothing beyond non-consensus, non-critical, non-crypto, etc. logic is being changed. The diff between the tests themselves would say a lot.