Add initial Augeas tests

[ferki]

This PR is a proposal related to #1609, and aimed at adding initial Augeas tests – including making the current implementation testable, and fixing an initial set of previously hidden bugs too.

The overall goal is to make it easier to maintain Rex::Commands::Augeas in the future by establishing its expected behavior.

In case you use Rex to interact with Augeas, please consider reviewing the changes, and leave notes or questions.

Checklist

• [x] based on top of latest source code
• [x] changelog entry included
• [ ] automated tests pass
• [x] git history is clean
• [x] git commit messages are well-written