First of all, locate the file DomainmodelGenerator.xtend in the package org.eclipse.xtext.example.generator

Workflow {
    ...
    component = Generator {
                ...
                // generator API
                fragment = generator.GeneratorFragment {
                    generateMwe = false
                    generateJavaMain = false
                }

package my.home.dsl.generator
import org.eclipse.xtext.generator.IGenerator
class DeepCloneGenerator implements IGenerator { ...
	override void doGenerate(Resource resource, IFileSystemAccess fsa) {...}
}

@RunWith(typeof(XtextRunner))
@InjectWith(typeof(DeepCloneInjectorProvider))
class DeepCloneGeneratorTest {
	
	@Inject IGenerator generatorDslToJava  // how to ensure DeepCloneGenerator implementation?
	@Inject ParseHelper<Model> parseHelper 
	
	@Test
	def test() {
		val code = '''
		package a.b.c
		a.b.c.OpeningHours {
			weekDay, from, to
		}'''
		val model = parseHelper.parse(code)
		val fsa = new InMemoryFileSystemAccess()
		generatorDslToJava.doGenerate(model.eResource, fsa)
		println(fsa.files)
		assertEquals(1, fsa.files.size)
		val topClonerJavaFileName = IFileSystemAccess::DEFAULT_OUTPUT + "Cloner.java"
		assertTrue(fsa.files.containsKey(topClonerJavaFileName))
		assertEquals('''
			package 
			public class OpeningHoursCloner {
			}'''.toString, 
			fsa.files.get(topClonerJavaFileName).toString
		)
	}
}

public class MyDslRuntimeModule extends org.xtext.example.mydsl.AbstractMyDslRuntimeModule {

	@Override
	public Class<? extends IGenerator> bindIGenerator() {
		// TODO Auto-generated method stub
		return MyGen.class;
	}
	
}

package my.home.dsl
public class DeepCloneRuntimeModule extends my.home.dsl.AbstractDeepCloneRuntimeModule {
	@Override
	public Class<? extends IGenerator> bindIGenerator() {
		return DeepCloneGenerator.class;
	}
}