docs: add caveat about calling "init" when using TestingModule

[verycosy]

Overview

I thought it would be helpful to add a note that when writing tests using NestJS’s TestingModule, you need to call the init method. Developers who aren’t familiar with the internal implementation of this package might not realize that the Nest application needs to be fully bootstrapped for things to work properly — just like I didn’t at first. 😅

PR Checklist

• [x] I read and included theses actions below

1. I have read the Contributing Guide
2. I have written documents and tests, if needed.