Null safety for getDescriptor.
Use standard RestartableJenkinsRule.
During restart tests, there could be two copies of a WorkflowRun in waitForCompletion at once.
This could lead to duplicated or mangled text being sent to the main log.
Check explicitly for Jenkins shutdown and break out of wFC in this case (also closing the log stream).
Bumping up core dependency to 1.572 to pick up NPE fix for SaveableListener.all().
Trying, so far without success, to make buildShellScriptAcrossRestart pass.
This would be the first test of DurableTaskStep that actually restarts while the task is running.
Can use ExtensionList.lookup now.
Merge branch 'master' into StepExecution

Conflicts:
	job/src/test/java/org/jenkinsci/plugins/workflow/job/SegmentTest.java
	job/src/test/java/org/jenkinsci/plugins/workflow/job/SingleJobTestBase.java