I have the default src/test/java folder for our unit tests. A separate folder src/integration/java is available for the integration tests.
If you only want to change the unit test source folder (rather than add an additional one), just change the testSourceDirectory element:
${project.basedir}/src/test/groovy
This is useful if all of your unit tests are written in groovy. (But you will also need to configure maven to compile your groovy code too - see the groovy-eclipse-maven-plugin or the build-helper-maven-plugin.)