ADEN-1359 Fix Synthetic mobile test cases

Please move the waitForElement methods into PageObject.
Perhaps consider moving these two into the verifyImgAdLoadedInSlot method.
Also, in verifyImgAdLoadedInSlot method, there is already an element lookup "WebElement slot = driver.findElement(By.id(slotName));"
Would "waitForElementById(slotName);" be necessary before the element lookup?