Tell devs to run the CLI command for setting up tests #33

The 'Getting Started' page for testing is a lot of creating folders, creating files, and copying code.

It'd be simpler and less room for error if you tell devs to run the CLI command ./craft tests/setup and then call out specific files they need to manually update. You could then remove the manual steps completely and just give a high-level overview of the folder structure.

Referencing: https://docs.craftcms.com/v3/testing/testing-craft/getting-started.html