Clarify Branching in Testing Plan

In our testing plan document, we should instruct the user to create a new branch off of development _before_ adding or pushing any commits, not after. This will prevent unnecessary merging.

Locate the testing plan document referenced in the issue and read the surrounding branching instructions. Update the order so users create a branch from development before adding or pushing commits, then review the rendered document to confirm the guidance is clear and avoids unnecessary merging.

Rédigé par le modèle d'indexation à partir du texte de l'issue.