Before this change, `index_html` for Catalog Tool was None that means trying to
access view for CatalogTool from UI always make a function call to the
` __call__` which function was overridden with `searchResults`, which
means everytime someone tried to access `portal_catalog/`, a useless query was
being made and an undesirable query result was displyed on the UI.

In this case, `index_html` is prefered whenever we try to access portal_catalog
from UI.

Also, add test for proper display of view when CatalogTool is accessed from XHTML UI

/reviewed-on nexedi/erp5!793