Add a test for Node.childNodes.

This test was initially written for Mozilla Servo by Tim Taubert; he gave
permission to submit it at
<http://krijnhoetmer.nl/irc-logs/developers/20140729#l-2043>.
Test that Node.childNodes always returns the same object.
Test that Node.childNodes contains non-element children as well.
Add a test for childNodes on a DocumentFragment and a Document.
Merge pull request #1126 from Ms2ger/childNodes

Add a test for Node.childNodes; r=zcorpan