After several rounds of trial-and-error with 'head', and tag 'workflow-1.1' I'm still not sure why tests fail. Currently I'm using Ubuntu 14.04 / JDK 1.8.0_25 (I had open-jdk7), and I've installed all the stuff that tests threw at me (e.g. mercurial). I assigned port 8081 to my local Jenkins test installation, and removed ~/.jenkins (which mvn hpi:run uses if it exists). Normally I use Eclipse following the Jenkins plugin development recommendations, but I also tried using Netbeans (8.02) (which brought even more problems).

Right now I still need to remove most of the tests from
cps/src/test/groovy/org/jenkinsci/plugins/workflow/cps/CpsScriptTest.groovy in order to get the workflow-plugin running.

Possibly there are system dependencies that my workstation doesn't meet. I know that the Java platform is supposed to be OS independent, but obviously it's not that simple. It would be very helpful if there were a list of requirements for the build- and test environment (Linux, BSD, Mac, Windows, distribution, installed software ... ).