shadow-dom: Update a test for encapsulation of Selector API.

This change adds a test that checks whether elements in a shadow tree are not
accessible from owner document's Selector API.

This change also removes an existing file that includes tests for Selector API.
This file includes tests for other document methods, but they are already
covered by dom-tree-accessors-001.html.
Merge pull request #225 from yutak/shadow-dom/selectors-api

shadow-dom: Update a test for encapsulation of Selector API.