[6.8] Experimenting with delay in test startup (#51459) (#52014)

* Experimenting with delay in test startup

* Move firefox tests sooner becuase they take a long time to run

* Move functional test parallel process delay to a spot that should work better for flaky test runner

vars/kibanaPipeline.groovy

@@ -11,6 +11,10 @@ def withWorkers(name, preWorkerClosure = {}, workerClosures = [:]) {
           nextWorker++
 
           return {
+            // This delay helps smooth out CPU load caused by ES/Kibana instances starting up at the same time
+            def delay = (workerNumber-1)*20
+            sleep(delay)
+
             workerClosure(workerNumber)
           }
         }