Ensure that we are tracking Go test coverage correctly

Created by: nicksnyder

From @keegancsmith on Slack

FYI the current measurement is much lower than the reality. Everytime we have tried to use -coverpkg it broke something (I think because of how it interacts with race? I can't remember). But right now we only collect coverage inside the package being tested, and miss out on coverage we get across packages. Fixing this might blow the OKR out of the water 🙂 So I'd investigate that first.

cc @unknwon since you are focusing on testing. I am going to tentatively place this in the next milestone because it isn't urgent.