If you use maven to run the tests (using the surefire plugin) then your output directory is set to target/surefire-reports. The test-output folder is something that is created by the TestNG eclipse plugin.
If you are using Maven to build an uber jar [ A jar that contains all its dependencies ] and then using java -jar <yourJar.jar> to run your tests, then you would need to ensure that you provide a mechanism to pass in the test output directory. I am guessing that in this case, you are using TestNG APIs.