Test TreeWalker with skipping
On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
Testing nextSibling
PASS walker.firstChild(); walker.currentNode.id is 'B1'
PASS walker.nextSibling(); walker.currentNode.id is 'B3'
Testing previousSibling
PASS walker.previousSibling(); walker.currentNode.id is 'B1'
PASS successfullyParsed is true
TEST COMPLETE