Using a separate testing mongo database on mongo and Heroku - mongodb

Question

In my project I am using https://www.npmjs.com/package/dotenv-safe in order to declare environment variables needed for configuration. For example:
NODE_ENV=development
JWT_SECRET=xxxxxxx
JWT_EXPIRATION_MINUTES=15
MONGO_URI=mongodb://mongodb:27017/proddb
BASE_URI=http://localhost:3000/
MONGO_URI_TESTS=mongodb://mongodb:27017/testdb
PORT=3000
Then I use those files in a config file :
module.exports = {
env: process.env.NODE_ENV,
port: process.env.PORT,
jwtSecret: process.env.JWT_SECRET,
jwtExpirationInterval: process.env.JWT_EXPIRATION_MINUTES,
mongo: {
uri: process.env.NODE_ENV === 'test'
? process.env.MONGO_URI_TESTS
: process.env.MONGO_URI,
},
logs: process.env.NODE_ENV === 'production' ? 'combined' : 'dev',
};
and in my package.json file, I've got:
"scripts": {
"start": "NODE_ENV=production node ./src/index.js",
"dev": "LOG_LEVEL=debug nodemon --inspect=0.0.0.0 ./src/index.js",
"test": "NODE_ENV=test nyc --reporter=html --reporter=text mocha --timeout 20000 --recursive src/tests"
}
The problem? Everything works fine but when tests are run on Heroku (prod) , they run on the main database and not on the testdb...