upgrader: make sure to keep erp5_upgrader itself by default

bt5/erp5_upgrader/SkinTemplateItem/portal_skins/erp5_upgrader/Base_getUpgradeBusinessTemplateList.xml

@@ -57,7 +57,7 @@
    a old business template without updating it and without removing it\n
 """\n
 \n
-return (\'erp5_base\',), []\n
+return (\'erp5_base\',), ["erp5_upgrader"]\n
 </string> </value>
         </item>
         <item>

bt5/erp5_upgrader_test/SkinTemplateItem/portal_skins/erp5_upgrader_test/Base_getUpgradeBusinessTemplateList.xml

@@ -50,7 +50,7 @@
         </item>
         <item>
             <key> <string>_body</string> </key>
-            <value> <string>return (\'erp5_web\',), []\n
+            <value> <string>return (\'erp5_web\',), ["erp5_upgrader", "erp5_upgrader_test"]\n
 </string> </value>
         </item>
         <item>