This restores previous behaviour to avoid breaking plugins that might
already extend or monkey-patch QUnit to add additional parameters to
the test callback.

Also:

* Declare the params setting in the Test class for added clarity,
  and to ensure a consistent object shape.

* Make more use of the new `addTest()` function that was added in
  #1569, this reduces a lot
  of duplication.

  While at it, I shifted the abstraction slightly to expose the
  Test class settings directly, thus making the `addTest()` mainly
  be responsible for the queuing and filtering, and no longer
  responsible for formatting the Test class settings.

  The use of ES2015 shorthand property name syntax makes feel
  almost identical to the function parameter signature.