Improve explanation for running tests

## Location within the Course

* Section: 4
* Lesson: 1
* Language: English

## Description

In the section "Run the Test Suite With setup.py", I found the instructions a bit confusing. It says: "run the test suite in a temporary testing environment, installing whatever is needed from test_requirements into that environment:". It wasn't immediately obvious to me that we should be in our projects/packaging/ folder, but in a new virtual environment where we `pip install` what is in the `test_requirements` list.

Perhaps a bit more explanation, plus a description of what else might be in the `test_requirements` list, if anything. Or if there are cases where we might want to add things to the list, if that applies at all. Since it's a list, seems like there might be multiple things in it.

Also, in the "Optional: Run the Test Suite Without setup.py", be more explicit that we are installing pytest in our original project package environment rather than a new test environment.

Commencez par la Section 4, Lesson 1, en particulier « Run the Test Suite With setup.py » et « Optional: Run the Test Suite Without setup.py ». Clarifiez le répertoire de travail, la distinction entre les environnements virtuels, l’installation de pytest, ainsi que le rôle et le contenu possible de test_requirements. Le travail sera considéré comme terminé lorsque les lecteurs pourront distinguer les deux workflows sans ambiguïté.

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