Updated controller.scatter.test.js to test default tooltip callbacks

[darless]

What changed

• This moves the mouse over the drawn point and verifies that there is
  no title in the tooltip and that the body contains expected.

Reason for change

• Initially implemented because I saw lacking code coverage for the default tooltip callbacks.

Testing

• Ran gulp test which passed both lint and unittests.
• Ran gulp unittest --coverage to verify that the associated lines for the controller.scatter.js are now covered.

[benmccann]

@Madrussian did you see test/specs/core.tooltip.tests.js? We have quite a bit of tooltip tests already and I think this is probably already covered by existing tests

I'm not sure that this test really belongs in controller.scatter.test.js. There's not really any tooltip-related code in the scatter controller. The location of the test code isn't corresponding to where the tooltip code is defined if we put it there, so I think it'd be better to add any additional tooltip test coverage to test/specs/core.tooltip.tests.js

[simonbrunel]

@benmccann https://github.com/chartjs/Chart.js/blob/master/src/controllers/controller.scatter.js#L26

[simonbrunel]

@Madrussian it totally makes sense to have it in controller.scatter.test.js since it's testing scatter defaults callbacks defined in the scatter controller. Just a minor change to make it simpler.

[benmccann]

But do we need to test the tooltip functionality again? Maybe we should just test that the default config matches what we expect? Like in https://github.com/chartjs/Chart.js/blob/master/test/specs/scale.time.tests.js#L54

[simonbrunel]

But do we need to test the tooltip functionality again?

We are not really testing tooltip functionalities but that the default options are correctly consumed.

expect(chart.options.tooltips.callbacks.title).toEqual(jasmine.any(Function));
expect(chart.options.tooltips.callbacks.label).toEqual(jasmine.any(Function));

Testing defaults as in the time scale doesn't guarantee that we don't break the current behavior, which is an empty title and (x, y) labels. We could manually call these methods and check their results, but how the test is currently written is more robust and as simple as just checking defaults.

[etimberg]

Left one minor comment. Am ok with the content of the test

[simonbrunel]

Thanks @Madrussian