Adding: firebase-jest-testing

**Link to project (GitHub)**: https://github.com/akauppi/firebase-jest-testing

**Describe the project in 2-3 sentences**:

Tools for testing Firebase back-end features, using Jest. This repo provides a "one stop", opinionated approach to testing Firebase projects. Using it may save you from reading countless pages of documentation and evaluating different testing strategies and libraries.

**Why would Firebase developers be interested in this project?**:

Setting up back-end testing of a Firebase project is not as easy as getting started with Firebase itself. This repo tries to navigate the treacherous waters and lead you through.

**Have you added a `.opensource/project.json` file to your site**:

No

Examples:
- https://github.com/invertase/react-native-firebase/blob/master/.opensource/project.json
- https://github.com/sgarciac/fuego/blob/master/.opensource/project.json
- https://github.com/firebase/emberfire/blob/master/.opensource/project.json

Revisa primero el repositorio firebase-jest-testing enlazado y los archivos de ejemplo .opensource/project.json. Después, inspecciona cómo firebaseopensource.com representa los proyectos enviados; se considera completado cuando el proyecto se añade usando el enlace al repositorio y la descripción proporcionados, con cualquier metadato requerido incluido.

Escrito por el modelo de indexación a partir del texto del issue.