By default, Maven runs your tests in a separate ("forked") process. You can use the maven.surefire.debug property to debug your forked tests remotely, like this:
mvn -Dmaven.surefire.debug test
The tests will automatically pause and await a remote debugger on port 5005. You can then attach to the running tests using Eclipse. You can setup a "Remote Java Application" launch configuration via the menu command "Run" > "Open Debug Dialog..."
If you need to configure a different port, you may pass a more detailed value. For example, the command below will use port 8000 instead of port 5005.
mvn -Dmaven.surefire.debug="-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000 -Xnoagent -Djava.compiler=NONE" test
You can force Maven not to fork tests by setting the configuration parameter forkCount to 0.
mvn -DforkCount=0 test
Then all you need to do is debug Maven itself. Since Maven 2.0.8, Maven ships with a mvnDebug shell script that you can use to launch Maven with convenient debugging options:
mvnDebug -DforkCount=0 test
Then you can attach Eclipse to Maven itself, which may be easier/more convenient than debugging the forked executable.