@firebase/testing - How to use auth custom claims in firestore rules tests?

I'm almost good with all my tests for Firestore Rules. But, I still need to test some path for the admin.

The admin in my app is not the Firebase admin, it's an user with privileges set like this in its customClaims :

claims: {admin: true}

How I can mock users customClaims with the npm package @firebase/testing ? I can't figure it out.

Thanks