Clean targets before launching tests using mvn verify

Tests launched using mvn verify do not remove the target directories first (as the clean goal is not specified), hence we end up with artifacts in Jenkins workspace that may not correspond to the last build.

The job https://qa.nuxeo.org/jenkins/job/FT-nuxeo-master-webdriver-dm-tomcat has already been changed to add the target "clean", but all tests using the mvn verify command should be updated too (not sure if there are potential unwanted side effects)