Document the easing curve tests

Update the test cases and add proper documentetation for easing curve tests.

The visual comparison for the curves can be done with the capacity tests. Running the state changes rapidly for different elements from left to right (or vice versa) causes the curve to be shown in the screen. And it is easy to compara that with the curve shape in the documentation:

https://doc.qt.io/qt-5/qml-qtquick-propertyanimation.html#easing-prop