Updated the test case with enabling fixed rate

Author: Manoranjan.Prakash

app/code/Magento/Checkout/Test/Mftf/Test/StorefrontGuestCheckoutTest/StorefrontGuestCheckoutWithSidebarDisabledTest.xml

@@ -18,6 +18,7 @@
             <group value="checkout"/>
         </annotations>
         <before>
+            <magentoCLI command="config:set {{EnableFlatRateConfigData.path}} {{EnableFlatRateConfigData.value}}" stepKey="enableFlatRate"/>
             <magentoCLI stepKey="disableSidebar" command="config:set checkout/sidebar/display 0"/>
         </before>
         <after>