Improve `dune runtest` output

[sim642]

Inspired by @michael-schwarz's rant, this issue is for possible ideas and discussions about improving dune runtest output from #1367.

• update_suite: instead of printing every executed test, print a dot (like ounit).
• Add prefix to update_suite No errors :) indicating the set of tests it's about.
• Investigate all "Excellent: ignored check"-s. Remove TODO only if passing thanks to analysis improvement. Otherwise improvement is unsound from a bug. #1425
• Add prefix to unit test output.
• update_suite: quiet output mode (-q?) to print nothing when successful, like cram tests.
• Grouping for GitHub Actions? https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#grouping-log-lines