fix: add newline back in

Author: jmcdo29

content/fundamentals/unit-testing.md

@@ -184,6 +184,7 @@ describe('CatsController', () => {
 ```
 
 You can also retrieve these mocks out of the testing container as you normally would custom providers, `modRef.get(CatsService)`.
+
 #### End-to-end testing
 
 Unlike unit testing, which focuses on individual modules and classes, end-to-end (e2e) testing covers the interaction of classes and modules at a more aggregate level -- closer to the kind of interaction that end-users will have with the production system. As an application grows, it becomes hard to manually test the end-to-end behavior of each API endpoint. Automated end-to-end tests help us ensure that the overall behavior of the system is correct and meets project requirements. To perform e2e tests we use a similar configuration to the one we just covered in **unit testing**. In addition, Nest makes it easy to use the [Supertest](https://github.com/visionmedia/supertest) library to simulate HTTP requests.