`mage test:all` should produce a combined code coverage result

[jzelinskie]

If we add the -coverprofile flag to ever go test func in magefiles/test.go, we can aggregate them all into a single coverage file in All().

This'll be useful to track (perfect coverage is a non-goal of the project), but is also required for submitting to awesome-go

[sephialaureenciaa]

Hi, i'm interested in contributing to this!

[tstirrat15]

@sephialaureenciaa thanks! have you checked out the CONTRIBUTING file? That should be all you need to get started and we'd welcome a pull request. If there's anything about the contribution guidance that seems like it's missing or confusing we'd also like to hear about it.