I was just working on my own plugin and ran into the same problem. I could not find documentation on it, but I had a look at the maven-ant-plugin (https://svn.apache.org/repos/asf/maven/plugins/trunk/maven-ant-plugin) to see how they did it.
I assume this is built-in test harness behavior, but you have to add a "project" parameter to your test pom.xml to set the value to a subclass of MavenProjectStub, for example in the "ant-test" test:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-ant-plugin</artifactId>
<configuration>
<project implementation="org.apache.maven.plugin.ant.stubs.AntTestMavenProjectStub"/>
<settings implementation="org.apache.maven.plugin.ant.stubs.SettingsStub"/>
<localRepository>${localRepository}</localRepository>
</configuration>
</plugin>
I hope you had your answer already, just posting for those to follow.