This is based in the recent tutorials from the great Todd Kerpelman on Youtube
So you can just follow the tutorials without writing, is a bit hard to follow without to much time.
First part Unit testing security rules with the Firebase Emulator Suite.
Second part Intermediate topics in Firebase Security Rules.
Steps to reproduce
Clone the repo, then don't be lazy and check the tutorials from Youtube :v
Start firestore emulator
firebase emulators:start --only firestore
Run tests
in the test directory run: npm test
Some notes
request.resource.data: The data who is been sending to db
resource.data: Data who is already in the db