Adjust acceptance tests to changes in @nextcloud/vue 2.6.3

Adjust acceptance tests to changes in @nextcloud/vue 2.6.3

The menu button and the menu of a share are no longer direct childs of
the actions of the share row. The menu button is now a child of a
".trigger" element, while the menu is a direct child of the body and has
an id defined in the "aria-describedby" attribute of the ".trigger"
element.

In XPath 1.0 it does not seem possible to "backreference" a value or
create variables, so when the share menu or one of its item is needed
now the ".trigger" element is first found and then its XPath expression
is used to compose its "aria-describedby" attribute in the XPath
expression for the menu.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>