[fix] widget.xml to match fixed class reference

Author: Marcel Hauri

app/code/Magento/Widget/Test/Unit/Model/_files/widget.xml

@@ -41,7 +41,7 @@
             </parameter>
             <parameter name="id_path" xsi:type="block" visible="true" required="true" sort_order="10">
                 <label translate="true">Product</label>
-                <block class="Magento\Backend\Block\Catalog\Product\Widget\Chooser">
+                <block class="Magento\Catalog\Block\Adminhtml\Product\Widget\Chooser">
                     <data>
                         <item name="button" xsi:type="array">
                             <item name="open" xsi:type="string" translate="true">Select Product...</item>