WIP: Clean up roulette testing

What does this MR do?

This is a refactor of the roulette.json handling code that runs as a Danger plugin. It extracts I/O calls to an external module so that they are easily testable.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

This could cause disruption to the reviewer roulette system, used during code review. This is a non-essential system, and the whole point of this MR is to mitigate any risk by improving tests and testability.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

This MR does not contain security relevant code.