¯\_(ツ)_/¯

Go to File > Open... > (navigate to build.gradle) > Open > Open as Project > check Use auto-import > Ok > voilà!

There are multiple ways to do it.

1. Command Line: Navigate to this project's root folder and run ./gradlew clean test.
2. IntelliJ IDEA - All tests: Open up the Project tab (Command+1) > right click the src/test/java folder > Run 'All Tests'.
3. IntelliJ IDEA - Single test: Right click an individual test class and then Run it.