I'm disappointed that almost everyone posting in this thread is making suggestions about what other people should do rather than thinking of ways to contribute themselves.
I plan to open at least one PR a month adding a useful new test to Bitcoin Core, or significantly improving an existing test, with the first PR to be opened by 31 October 2018.
What do the rest of you plan to do?
I plan to continue working on a competing implementation to Bitcoin Core. It was because of Bitcoin Unlimited that this bug was caught, when Awemany noticed it while working on the consensus changes for the November fork in BCH. This tells me that multiple implementations and competing development teams is a good thing.