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.

从第 4 节、第 1 课开始，重点关注“Run the Test Suite With setup.py”和“Optional: Run the Test Suite Without setup.py”。明确工作目录、虚拟环境之间的区别、pytest 的安装方式，以及 test_requirements 的用途和可能包含的内容。完成的标准是读者能够毫无歧义地区分这两种工作流。

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