This page discusses the steps to configure a plugin project for creating tests.
Create a Folder for Tests
Open the plugin project and create a separate folder named test under the src directory. Under test, create the java folder for test source code, and the folder testData for test data files and reimport the Gradle project.
Set the Run Configuration Parameters
Because some tests use Java files as test data, the tests need to mock up the project's SDK. See the How to test a JVM language? section for details.