I needed to move some src/test/java to src/main/java according to this recommandation from maven-jar-plugin documentation => http://maven.apache.org/plugins/maven-jar-plugin
Since the test classes have now been placed in src/main/java, it is likely that they are getting compiled during the compile phase to target/classes.
So, though surefire is configured to indicate that the test source location is src/main/java, it would still look for test classes in target/test-classes, since this is the default location for test classes.
You could try specify testClassesDirectory parameter suitably and see if it works.
Also, I think you may have misinterpreted the recommendation in the maven jar plugin documentation. I do not believe they suggest moving test sources to src/main/java.