prints test class FQDN name and its duration in seconds

getTestClassesDuration.groovy

import hudson.tasks.test.AbstractTestResultAction;
import hudson.tasks.junit.ClassResult;
import hudson.tasks.test.TabulatedResult;
import hudson.tasks.test.TestResult;

def jenkins = Jenkins.instance
def job = jenkins.getItem("your job")
def buildNum = 123
def bld = job.getBuildByNumber(buildNum)
//println bld.actions
def testResults = bld.getAction(AbstractTestResultAction.class)
def testData = testResults.getResult()
println testData instanceof TabulatedResult
TabulatedResult tr = (TabulatedResult) testData;
for (TestResult child : tr.getChildren()) {
   for( hudson.tasks.junit.ClassResult cr: child.getChildren()){
     println "${child.packageName}.${cr.className}, ${cr.duration}"
  }
}