JobUtils.delay method problem in core tests

JobUtils.delay method checks if Display.getCurrent() renurn not null value
and use returned display to delay hold execution. If value is null it
uses Thread.sleep(). Problem is whwn tests are exeuted in core mode without
WorkbenchUI Display.getCurrent() stil returns not null value and sleep
method for returned object can hold execution for a long time.

Check if workbench is running added before trying to get current display to
avoid this problem