jquery.mwExtension.test: Use better sample data for isDomElement

jquery.mwExtension.test: Use better sample data for isDomElement

* getElementById, getElementsByTagName()[0] and jQuery#get(0) all
  return an HTMLElement. Test that only once.
  Avoid using random internal QUnit elements like #qunit-header.
  These don't always exist (e.g. when running tests via Karma).
  If anything, it should use #qunit-fixture. But might as well
  create a simple DIV and test that.

* Use 'null' instead of getElementById('not-found'). We're
  testing isDomElement, not getElementById.

* Add test for TextNode.

* Separate tests for isDomElement and isEmpty.

Change-Id: If9284fbbd066897561ae8e679f5f7ceab9255e18