Using forName to retrieve a class in the test module via main

In my main module, I have some code which uses forName(). In a JUnit test I've created a dummy class which I'm trying to retrieve from the main module.

public class TestClass {
    public static class DummyClass implements OtherClass { ... }

@Test
void testCase() {
    ParserHandler parserHandler = new ParserHandler();
    String fullClassName = DummyEntity.class.getCanonicalName();

    // fullClassName is used in the following method which uses forName()
    parserHandler.run(fullClassName);
}

Whenever I try this, I get:

java.lang.ClassNotFoundException: com.x.y.TestClass.DummyEntity

How can I resolve this?