Every IRIS plugin gets unit tests auto-generated as part of the Plugin SDK. The auto-generated test simply tests retrieving the plugin's application context. It does this by bootstrapping IRIS using the class com.sri.iris.test.IrisJUnitTestCase. See the Plugin Tutorial for more information on adding plugin test cases.
It is also possible to add more unit tests for a plugin by creating source files ending with "_T.java" (where the classes inherit from JUnit's TestCase or the IrisTestCase instance for the plugin if the test needs to bootstrap IRIS).
Unit tests for core IRIS functionality exist under iris/test. These unit tests only apply to IRIS sourec code in iris/src. To add a unit test, simply create a java class with the suffix "_T.java" which inherits from JUnit's TestCase or com.sri.iris.test.IrisTestCase (if the unit tests needs to bootstrap IRIS).