TestFramework: Add gstest_to_junit_xml.py.

[ivucica]

This is currently unused in gnustep-tests, and it is not installed. The intention is to have a representation of tests run under gnustep-tests in the JUnit test report XML format. This is a format ingested and understood by many utilities, so having a way to trigger gnustep-tests to produce it would be beneficial.

Ideally, the framework itself would produce much more structured output, that is easier to parse; or it would support creating the .xml natively. The next best thing would be to process tests.log; however, the simplest approach was to process tests.sum. This can then trivially be plugged into gnustep-tests, controlled by an environment variable.

The prototype is done in Python 3, specifically under Python 3.11. Switching to Objective-C does not necessarily introduce a dependency loop, as it is perfectly valid to run tests only after GNUstep Base has already been installed.

Issue: #53 Bug: 53

[rfm]

I detest the bloated nature of junit, but I do recognise that it's widely used/supported, and that feeding test results into external tools could be useful, so I don't see any problem with this approach.