Refactor selectNodes tests

* Move them from ve.example.js to ve.dm.example.js
** Also move lookupNode() and createDomElement() there
** Delete ve.example.js because there's nothing left in it
* Make main example document implicit, but allow override
* Specify nodes as arrays and do the lookup later
* Specify range and mode separately rather than calling selectNodes()
  during construction
* Construct statically rather than in a function
* Use expect( cases.length );

Change-Id: I620e949c5e612b32eaa57c5d9b60cc91f9ddbf02