Add tester for Card component

[ugur-vaadin]

The Card component was introduced in v24.7 under a feature flag. There is not much interactivity with the component, therefore I am not sure if it needs any helpers.

[TatuLund]

Card implements HasComponents, hence $ query should find its children I think.

Card has specific getters for the slotted children.

The above should cover the testing needs.