Fail to test a maven plugin - maven

Question

I've developed a maven plugin and would like to unit test it. According to Maven's website I should use the testing harness https://maven.apache.org/plugin-testing/maven-plugin-testing-harness/getting-started/index.html
But after setting up that simple test project I get an exception.
org.codehaus.plexus.component.repository.exception.ComponentLookupException: java.util.NoSuchElementException
role: org.apache.maven.plugin.Mojo
roleHint: org.tbee.xml2xxx:xml2yaml-maven-plugin:0.2.0:convert
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:267)
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:243)
at org.codehaus.plexus.PlexusTestCase.lookup(PlexusTestCase.java:205)
at org.apache.maven.plugin.testing.AbstractMojoTestCase.lookupMojo(AbstractMojoTestCase.java:410)
at org.apache.maven.plugin.testing.AbstractMojoTestCase.lookupMojo(AbstractMojoTestCase.java:355)
at org.tbee.xml2yaml.maven.plugin.XML2YAMLMojoTest.testSomething(XML2YAMLMojoTest.java:52)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at junit.framework.TestCase.runTest(TestCase.java:177)
at junit.framework.TestCase.runBare(TestCase.java:142)
at junit.framework.TestResult$1.protect(TestResult.java:122)
at junit.framework.TestResult.runProtected(TestResult.java:142)
at junit.framework.TestResult.run(TestResult.java:125)
at junit.framework.TestCase.run(TestCase.java:130)
at junit.framework.TestSuite.runTest(TestSuite.java:241)
at junit.framework.TestSuite.run(TestSuite.java:236)
at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:90)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:89)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:41)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:542)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:770)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:464)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)
Caused by: java.util.NoSuchElementException
at java.base/java.util.Collections$EmptyIterator.next(Collections.java:4210)
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:263)
... 24 more
From what I can debug the DefaultPlexusContainer is searching for the correct GAV, but I cannot find that it actually loaded the project-to-test pom.
Using the rule based version also fails, first because the Rule.getTestFile method does not exist. But replacing that with normal file access, it does load the project-to-test pom, it also derives the GAV for the plugin from the project's pom, so far so good. But then calling the lookup method ends up and fails in the same DefaultPlexusContainer.locate method as the previous test approach.
org.codehaus.plexus.component.repository.exception.ComponentLookupException: java.util.NoSuchElementException
role: org.apache.maven.plugin.Mojo
roleHint: org.tbee.xml2xxx:xml2yaml-maven-plugin:0.2.0:convert
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:267)
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:243)
at org.codehaus.plexus.PlexusTestCase.lookup(PlexusTestCase.java:205)
at org.apache.maven.plugin.testing.AbstractMojoTestCase.lookupMojo(AbstractMojoTestCase.java:410)
at org.apache.maven.plugin.testing.AbstractMojoTestCase.lookupMojo(AbstractMojoTestCase.java:355)
at org.apache.maven.plugin.testing.MojoRule.lookupMojo(MojoRule.java:164)
at org.tbee.xml2yaml.maven.plugin.XML2YAMLMojoTest.testSomething(XML2YAMLMojoTest.java:60)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.apache.maven.plugin.testing.MojoRule$2.evaluate(MojoRule.java:308)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:89)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:41)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:542)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:770)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:464)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)
Caused by: java.util.NoSuchElementException
at java.base/java.util.Collections$EmptyIterator.next(Collections.java:4210)
at org.codehaus.plexus.DefaultPlexusContainer.lookup(DefaultPlexusContainer.java:263)
... 33 more
Maybe it has to do with that I use Java 11? I cannot find a clear statement what is the maximum Java version to use for plugins.
The plugin works fine BTW. Not being able to unit test it, does not mean it cannot be used.