How to add a file to IntelliJ plugin project test for iterating in the plugin

I'm trying to test an IntelliJ plugin which iterates over content in the project using ProjectFileIndex.SERVICE.getInstance(project).iterateContent. How do I load a Java file to the test's project so that it will be iterated over?

I've tried myFixture.configureByFile("TestData.java") which adds a Java file at /src/TestData.java but I only see the plugin iterate over /src and nothing below it.