Adds a workflow for running spread and writing the test results to a JUnit XML test file. This should be used by any craft workflow that run spread. While working on https://github.com/canonical/starflow/pull/209, I was effectively creating a bespoke test report format. I decided to extract it into it's own script. Standardized test reports are useful for scenarios such as: - Test failures are more readable for humans when displayed in a summary ([example](https://github.com/dorny/test-reporter/blob/main/assets/fluent-validation-report.png)). - Agents can parse them more reliably and efficiently. - It paves the way for a workflow that retries failing tests. I chose JUnit as the format because it's widely supported and well-known. This is a heavily LLM-assisted PR, as this isn't production code. Tested downstream in https://github.com/canonical/snapcraft/actions/runs/42112040217?pr=6545 On a related note, [spread-plus](https://github.com/canonical/spread-plus#json) has a `-json` flag for exporting its own bespoke test report. If that gets upstreamed to spread, we can switch to it.