[issue30934] Document how to run coverage for repository idlelib files.

[Louie Lu]

Louie Lu added the comment:

I update how to prepare coverage on Linux and MacOS. I first thought user will prepare as Terry wrote at the top of `5. Test Coverage`. 

Cheryl's method is good, the update guide is borrow from here. First create a virtualenv, then install coverage via pip in venv. If user want to skip the step of venv, they can copy out the coverage package in venv/lib/ to Lib/ inside.

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