The test-harness tries to use the directory jenkinsTests.tmp inside the system temp directory when running (see https://github.com/jenkinsci/jenkins-test-harness/blob/master/src/main/java/org/jvnet/hudson/test/TemporaryDirectoryAllocator.java#L53).

While this might even open the user for an attack (I'm currently not sure if temp file attacks also are a problem for directories or only for files), this also prevents running the harness from multiple users on the same machine.

I would suggest randomizing part of the name of this directory.