BS-10185 : Fix unstable test org.bonitasoft.engine.event.TimerEventTest.timerStartEventDuration

Rewrite the test.
Replace :
  "List<ProcessInstance> processInstances = getProcessAPI().getProcessInstances(0, 10, ProcessInstanceCriterion.CREATION_DATE_DESC);
  assertTrue(processInstances.isEmpty());

  // wait for process instance creation
  Thread.sleep(4000);

  processInstances = getProcessAPI().getProcessInstances(0, 10, ProcessInstanceCriterion.CREATION_DATE_DESC);
  assertEquals(1, processInstances.size());"

by "waitForInitializingProcess();"