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