chore: pre-factor code related to junit reporting

[porridge]

What this PR does / why we need it:

This is preparation for changes needed to solve #575 .

There are no significant changes to functionality. See the individual commits, which:

1. change the report ordering to be stable (name-based) - this makes the subsequent test simple. I was somewhat surprised that the top-level (suite) order can differ, even though it's not executed in parallel. I don't think this is due to concurrency, but I'm adding a mutex just in case.
2. add a dummy test suite and golden files to prevent unintended changes to the reports
3. introduce an abstraction for reporting, that in a future PR will internally be creating the report slightly differently, based on a configuration setting