Bug 1080764 - Add 'anon' and 'anon attribute' search strategies to marionette.find_element(), r=AutomatedTester

The 'anon' search strategy maps directly to nsIXULDocument.getAnonymousNodes() whereas the 'anon attribute'
strategy maps to nsIXULDocument.getAnonymousElementByAttribute(). These strategies are needed for clients who
wish to find and manipulate anonymous content, typically found in the Firefox chrome. For more details, see:
https://developer.mozilla.org/en-US/docs/XBL/XBL_1.0_Reference/Anonymous_Content

--HG--
rename : testing/marionette/client/marionette/tests/unit/test_switch_anonymous_content.py => testing/marionette/client/marionette/tests/unit/test_anonymous_content.py