Summarize test failures in the CI job summary and annotations - #2791

Failures in the CI step logs have gotten hard to find, the unit test step alone is ~35k lines for a single failure. junittool report lists each failed test with just its t.Error / t.Fatal output in the job summary and as annotations.

• go test only marks t.Error output in -json without -v, so the gotestsum runs drop -v
• the unit test logs print one line per package plus failed test output, the full output is still in the uploaded json
• the unit test JUnit files are registered now, so verify actually checks them

NOTE: There are some off the shelf tools, but we already had one, so tests aside this isn't much code and avoids taking another third party action dependency, while keeping our existing junit collection etc. It also folds a parent test into its failed subtests, names the tests that never finished on a timeout, and points panic annotations at the right line.

Tested locally with sample failures, against the artifacts from a failed main run, and on this PR with a since-dropped commit of failing unit tests.

AI-assisted.

• Tests pass
• Appropriate changes to documentation are included in the PR