Share some more code in the DOMImplementation#createHTMLDocument test.
Share code between DOMImplementation#createHTMLDocument tests in DOM and HTML.
Introduce variables to clarify DOMImplementation-createHTMLDocument.html.
Add some more checks to DOMImplementation-createHTMLDocument.html.

These checks are inspired by the relevant Servo test.
Merge pull request #1311 from Ms2ger/createHTMLDocument

Cleanup and improve DOMImplementation#createHTMLDocument tests; r=zcorpan