Laravel provides fully covered online documentation.
But if you are still wondering, how to run PHPUnit tests in Laravel 5.5 with testing environment config.

It’s quite simple:
The proper way is to create testing environment file name it .env.testing. In this file you can define testing configuration including database settings.

DB_CONNECTION=mysql
DB_HOST=127.0.0.1
DB_PORT=3306
DB_DATABASE=db_testing
DB_USERNAME=testing_user
DB_PASSWORD=testing_user_password

Then run following commands:

• php artisan config:cache –env=testing
  (clear config cache for testing)
• php artisan migrate:refresh –env=”testing”
  (prepare test database)
• php artisan migrate:refresh –seed –env=”testing”
  (prepare test database & run seeder if you have created any seeders)

Laravel 5.5 official documentation link.