ctest_summary.py renders a Test.xml as markdown for
GITHUB_STEP_SUMMARY: a one-liner when everything passes, otherwise the
first few failures inline with a link to the full Test.xml on MinIO.
It's also usable locally with --all to list every failure from a
downloaded Test.xml.
The coverage job now also generates and uploads gcovr's html-details
report and links it from the step summary.
