Fix createSlave for RestartableJenkinsRule

If you use JenkinsRule.createSlave (or createOnlineSlave) from a test based on RestartableJenkinsRule, it seems to work during the first session. But then the whole slave directory gets quietly deleted during the restart.

Either this deletion should be suppressed, or there should be an alternate method to create a slave that actually survives the restart intact.

See WorkflowTest.buildShellScriptAcrossRestart and BindingStepTest.cleanupAfterRestart.