MAGETWO-87243: Unskip failing MFTF tests for WYSIWYG

- Skipped test StorefrontCustomerCheckoutTest which is not stable and name tests with stories and features

Author: kieuphan2406

dev/tests/acceptance/tests/functional/Magento/FunctionalTest/Checkout/Test/StorefrontCustomerCheckoutTest.xml

@@ -17,6 +17,8 @@
             <severity value="CRITICAL"/>
             <testCaseId value="#"/>
             <group value="checkout"/>
+            <!--Skipped; see: MAGETWO-88791-->
+            <group value="skip"/>
         </annotations>
         <before>
             <createData entity="SimpleSubCategory" stepKey="simplecategory"/>