CI: Use native ERT JUnit Reporting

[phairoh]

I originally couldn't get native JUnit reporting to produce a file, so I fell back to ert-junit, which is unmaintained and crashes when a test errors, so test-results.xml was never generated on those runs. As it turns out native JUnit was fine all along, I just wasn't setting the output name via ert-load-file-name (the EMACS_TEST_JUNIT_REPORT env var enables it but doesn't control the filename). This PR drops ert-junit and uses the built-in reporter.

Also, in test-report.yml, dorny/test-reporter@v3 defaults use-actions-summary: true, which doesn't attach the test report to the commit so you have to go digging for it. Setting it to false fixes this. NOTE: this will only take effect once it's on main.

What Passing tests will look like when you click on the checkbox next to the commit:

Failing tests:

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I'm making visual changes, so I'm including screenshots so you can view and discuss.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[xenodium]

Lovely! Thank you for looking into it.