Fixing random test failures with Slider and DoubleSlider

This PR makes `aria.utils.dragdrop.Drag` a static dependency of the Slider and DoubleSlider widgets. It does not make sense to have it as a dynamic dependency, as it is always needed, as soon as those
widgets are used. Moreover, using a dynamic dependency makes their test fail randomly, as the dependency may not have been completely loaded when the test starts, leading to errors.