Add a test file for local bookmark.(#text1, #text2, #text3, #top, #h3, #num)
Implement local bookmark(hashtag) for Acid2. With @joonwonlee

Support hashtag through link (e.g. <a href="#top">)
auto merge of #1262 : june0cho/servo/acid2_local_bookmark, r=jdm

To test this, execute the followings:
    test_local_bookmark.html#text1
    test_local_bookmark.html#text2
    test_local_bookmark.html#text3
    test_local_bookmark.html#top
    test_local_bookmark.html#h3
    test_local_bookmark.html#num
    acid2.html#top
     - The link from acid2.html works now. ( `<a href="#top">`)

Thanks to @metajack!
Ensure that it's safe to modify the DOM node pointers before doing so. Fixes #1224.
Add forgotten test for Element.attributes.
auto merge of #1252 : jdm/servo/layoutassert, r=jdm,metajack

...224.