Add testing best practices to 'Read and write files' page

[fmardini]

Page URL

https://docs.flutter.dev/cookbook/persistence/reading-writing-files/

Page source

https://github.com/flutter/website/tree/main/src/cookbook/persistence/reading-writing-files.md

Describe the problem

It's not very clear what are the best practices to test this code, would be great if you could add a testing section

Expected fix

No response

Additional context

No response