[test] Fix coverage report introduce by f0fa155a83
[test] Fix coverage report introduce by f0fa155a83

- in order to avoid coverage report on playground code, I changed
overalls to run test on `pkg` instead of `.`, which result in `overalls.coverprofile`
generated in `pkg` folder, so goveralls can't find
`overalls.coverprofile` because it is not in project root

xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k