Fix paste tests to work in phantomJS

PhantomJS doesn't play nicely with document.execCommand and complex
HTML. For testing the attribute stripping, use the clipboardData
path. Add in a special clipboard key value that allows us to manually
trigger this from the tests ('useClipboardData-0').

Change-Id: I0de842095a6a9a766f0f9a2249064ca7707283c8