I guess the natural follow-up question to that is: where and how can we improve test coverage in Base? Which then loops back to the original topic of this thread (how to make contributions flow more easily).
Here I’m optimistic because it seems other language communities have faced the same challenge and come up with a number of ideas the Julia community could take inspiration from, e.g. quest issues as mentioned in What's the state of Automatic Differentiation in Julia January 2023? - #19 by jlapeyre. As long as there’s institutional support for such initiatives (for however much core Julia development can be considered an “institution” 
), the road ahead could be promising.