Added purchaseOrderNumber element in CheckoutSuccessMainSection

Author: shanthi

app/code/Magento/Checkout/Test/Mftf/Section/CheckoutSuccessMainSection.xml

@@ -21,5 +21,6 @@
         <element name="printLink" type="button" selector=".print" timeout="30"/>
         <element name="orderNumberWithoutLink" type="text" selector="//div[contains(@class, 'checkout-success')]//p/span"/>
         <element name="orderLinkByOrderNumber" type="text" selector="//div[contains(@class,'success')]//a[contains(.,'{{orderNumber}}')]" parameterized="true" timeout="30"/>
+        <element name="purchaseOrderNumber" type="text" selector="div.checkout-success > p:nth-child(1) > a span"/>
     </section>
 </sections>