Add a section for overriding APP_* token providers

[davids-sparkshare]

This adds a section on how to replace APP_* token providers in testing. This follows the description by @kamilmysliwiec in nestjs/nest#4053 . I've used the solution in my project and it worked great.

Thanks to @jmcdo29 who pointed me to that issue when I was struggling with this.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[x] Docs
[ ] Other... Please describe:

What is the current behavior?

Issue Number: nestjs/nest#5821

What is the new behavior?

more docs

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

Can you review @jmcdo29 by chance? 🤞

[davids-sparkshare]

Applied all suggestions from code review.

[kamilmysliwiec]

LGTM