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.