If you want simply to have the jenkins.war as fast as possible (without test
execution), run:

    mvn clean install -pl war -am -DskipTests

The WAR file will be in war/target/jenkins.war (you can play with it)
You can deactivate test-harness execution with -Dskip-test-harness

For more information on building Jenkins, visit
https://wiki.jenkins-ci.org/display/JENKINS/Building+Jenkins

Have Fun !!