MC-5764: Custom customer address attribute does not display during add address template of checkout with Multiple address flow.

- Added supporting test code

Author: nathanjosiah

app/code/Magento/Multishipping/Test/Mftf/Page/NewMultishippingAddressPage.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+ /**
+  * Copyright © Magento, Inc. All rights reserved.
+  * See COPYING.txt for license details.
+  */
+-->
+
+<pages xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xsi:noNamespaceSchemaLocation="urn:magento:mftf:Page/etc/PageObject.xsd">
+    <page name="NewMultishippingAddressPage" url="/multishipping/checkout_address/newShipping" module="Multishipping" area="storefront">
+    </page>
+</pages>