generate coverage reports

[@dnephin commented in @302](https://github.com/gotestyourself/gotestsum/issues/302#issuecomment-1513993027) in a discussion about coverage output:
> go tool cover (https://pkg.go.dev/cmd/cover) is a similar idea. It can produce 2 reports: coverage percentage per function, and a detailed HTML report. The HTML report effectively has this information, but it's not presented this way.
>
> It seems like it wouldn't be too difficult to copy and modify https://cs.opensource.google/go/go/+/refs/tags/go1.20.3:src/cmd/cover/html.go to make a new report in the format you want. It is possible there is a tool out there that has already done the work, I haven't looked. If you want to build it, it could work as gotestsum tool cover-report or something similar. A [new issue](https://github.com/gotestyourself/gotestsum/issues/new) for this would be great.

I use coverage reporting in CI, and [GitLab's coverage visualization](https://docs.gitlab.com/ee/ci/testing/test_coverage_visualization.html) requires cobertura format. I'm currently using [gocover-cobertura](github.com/boumenot/gocover-cobertura) to generate this report. It would be handy to be able to generate both the junit and cobertura reports from gotestsum all at once -- possibly alongside the html output that was mentioned in the comment that dnephin was replying to.

Review gotestsum's existing JUnit reporting path and the Go cmd/cover/html.go reference mentioned in the issue. Compare the requested Cobertura output with gocover-cobertura and GitLab's coverage visualization requirements. Done means gotestsum can generate JUnit and Cobertura reports together, with the scope of HTML output clarified.

Written by the indexing model from the issue text.