Moves the ListAppender class from codjo-agent to codjo-test

## Context
The `ListAppender` class wasn't specifically related to `codjo-agent` but was part of it.

## Description
- Moves the `ListAppender` class from `codjo-agent` to `codjo-test`.
- Adds `LoggerRule`, an implementation of JUnit's `Rule` to ease usage of `ListAppender`.