L5 doesn't use the .env.testing.php notation anymore.
In your root directory you should notice a file .env.example or .env If it happens to be the file to .env.example, you should manually rename it to .env
Anyhow, in this file you can put all of your environmental variables.
When you open the file you will see something like
APP_ENV=local
APP_DEBUG=true
APP_KEY=SomeRandomString
DB_HOST=localhost
DB_DATABASE=homestead
DB_USERNAME=homestead
DB_PASSWORD=secret
CACHE_DRIVER=file
SESSION_DRIVER=file
MAIL_DRIVER=smtp
MAIL_HOST=mailtrap.io
MAIL_PORT=2525
MAIL_USERNAME=null
MAIL_PASSWORD=null
Now, in L5 you can create a file .env.testing and put your testing configurations in. Make sure that yout APP_ENV equals to testing; therefore, in your .env.testing file you should have:
APP_ENV=testing