How to use Laravel facades (Cache, Log, Storage) in package outside Laravel

Please point out any naivete or incorrect assumptions I'm making about Laravel, Composer, PHPUnit, etc.

I had a class called SpeechToTextHelper that was inside a Laravel project, and it used facades like this:

use Cache;
use Log;
use Storage;

Then, since I wanted to share it between multiple Laravel projects, I moved it into a separate repo and required it (into the first project) as a dependency via Composer.

The code all seems to run fine.

My question is different from Using Laravel Facades outside Laravel

What I want to know is:

1. Now that I also want to write PHPUnit tests for SpeechToTextHelper in my new tools repo, I see errors like RuntimeException: A facade root has not been set. and Error: Class 'Log' not found, presumably because this tools repo has no awareness of Laravel. I guess this means my production code has been working just by side-effect.
2. In my new tools repo (where my SpeechToTextHelper now is), how am I supposed to indicate (maybe somewhere in composer.json?) that the code will only work if Laravel's facades exist and are initiated properly?

How can I fix my separate repo's code so that its tests can run and also so that it ensures that it can only be "required" by a Laravel project?

via Chebli Mohamed