Add a test for template contents handling in importNode

Add a test for template contents handling in importNode, I missed this as part of:

https://codereview.chromium.org/173303004/

Review URL: https://codereview.chromium.org/180293002

git-svn-id: svn://svn.chromium.org/blink/trunk@167871 bbb929c8-8fbe-4397-9dbb-9b2b20218538