There are two strategies to write JUnit test for JSF applications. The first strategy uses a tool like Arquillian to start both an application server and a browser that runs a JSF application. Yeray Santana Borges used this strategy to contribute a couple of JUnit tests to BootsFaces.
The big disadvantage is performance. It takes a while to start an application server. That’s bad if you want to run hundreds of JUnit tests on every build. Usually projects prefer to define a dedicated Maven profile to run this kind of tests. By default, the tests are skipped by the build. They are run only during a nightly build.