Add test report to the GitHub actions workflow.

[manud99]

I noticed that the GitHub workflow does not produce any useable output that can be analyzed. This makes it difficult to find out why some tests fail without running the entire test suite on your machine for approx. 10-15 minutes. To solve that I extended the GitHub Actions workflow:

• I added a step which generates a nice test report using Markdown that lists all test suites and failed test cases together with a short summary. Under the hood it uses the repository phoenix-actions/test-reporting.
• I added a step which uploads the test reports in XML format as an artifact. This way you can download and inspect the reports manually if needed.

An example of such a test report can be found here.