Where I work there is a system made many years ago in Play 2.0.4 and works very well so far.
I would be interested to know how to add unit tests, what libraries are needed.
Also, if possible then use a continuous integration server to automate the build.
If you have experience in that, I thank you very much.
To add unit test cases, the Play documentation contains a lot of useful info to get you started, https://www.playframework.com/documentation/2.0.x/JavaTest
Essentially you will be using JUnit, assertj and various mocking libraries such as mockito. awaitility is also very useful for testing async codes.
As for continuous integration, we use Atlassian Bamboo (with some plugins and scripts) to automatically build, test, deploy our applications to the Cloud, and it works beautifully, as Play at the end of the day, is still a Java application. I am sure you can use other build systems to achieve the same thing.