Trac-28709: QUnit Tests

Here are some more `QUnit` tests for `customize-base.js`. They test a constructor being passed to `wp.customize.Class.extend`. All of the tests pass.

Though this was a todo, it seems the main need is for unit tests of customize-controls.js and customize-widgets.js. This is noted in the [todos](https://github.com/xwpco/wordpress-develop/pull/29) as "Add unit tests for JS models."

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