Gave up on slather because it had some major bugs that need fixing. And Caro brought up a good point about trying to limit the number of dependencies.
So instead of slather, Apple's xccov is being used. The generated report is rather plain but as services like codecov.io start adopting the new format, I think we should look into using their services for making the reports look more comprehensible.
Here's a dummy CircleCI build with some test code in it.