Fix testing_routes_and_controllers sections

[guillaume-giannantonio]

I just added another section to separate the "Learning outcomes" part from the actual lesson. It wasn't making sense to have the full lesson in this part + when you were scrolling you would still see that you're reading the lesson from "Learning outcomes part".

I'm not quite sur about the title that I gave though ("Routes testing with supertest").

Because

"Learning outcomes" should not contains the entire lesson.

This PR

I just added a title to separate sections.

Issue

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Additional Information

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Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[ManonLef]

Good catch, thanks for adding this!