MAGETWO-87243: Unskip failing MFTF tests for WYSIWYG

- Skip test AddVariableToBlock

Author: kieuphan2406

dev/tests/acceptance/tests/functional/Magento/FunctionalTest/Cms/Test/AdminAddVariableToWYSIWYGBlockCest.xml

@@ -12,6 +12,8 @@
             <features value="MAGETWO-36659-[CMS] WYSIWYG update"/>
             <stories value="MAGETWO-42158-Variable with WYSIWYG"/>
             <group value="Cms"/>
+            <group value="skip"/>
+            <!-- Skipped; see MAGETWO-88218 -->
             <title value="Admin should be able to add variable to WYSIWYG content of Block"/>
             <description value="You should be able to add variable to WYSIWYG content Block"/>
             <testCaseId value="MAGETWO-84378"/>