build: Report coverage inline in GitLab reports
- Don't skip reporting of files that are covered.
- Don't over-ride the reports inline, but use the .nycrc.json file's spec.
- Replace lcov report label with 'html' (same thing in practice).
- Add 'cobertura' reporter format, which GitLab likes apparently.
- Steal bd808's 'coverage' regex line from wikibugs2 for inline reporting.