Limit playground references to tests

[Arthur-Milchior]

The system used by https://google.github.io/comprehensive-rust/exercises/day-1/book-library.html allows to execute code directly in the book. So playground references became useless. If I got it wrong, it would be nice to at least explain why it’s not sufficient to edit and run in this book.

The only exception I saw is in the test setting. Playground allows to run tests. In this case, I believe adding a sentence warning that tests are ignored by this book running system means that we really must use the playground and ignore the "run" button provided by the book