Add a Tester class for VirtualList component

[mshabarov]

Create a Tester class for VirtualList

[joelpop]

What methods should VirtualListTester support?

Because VirtualList is really just a display component (non-interactive), there won't be any methods to set, clear, or select things. Looking at the non-interactive methods of GridTester for inspiration, candidate methods to consider supporting are:

• int size()
• T getItem(int index)
• String getItemText(int index)
• Component getItemComponent(int index)
• <V> V getLitRendererPropertyValue(int index, String propertyName, Class<V> propertyClass)
• void invokeLitRendererFunction(int index, String functionName)

In order to support the latter two, VirtualList must be changed to implement and publicly expose a getRenderer method as Grid.Column does.

Please provide your thoughts on what methods should or shouldn't be provided by this tester, and if you have alternate suggestions for their names and/or parameter names.

[MatthewVaadin]

At least for the use cases we require, the last two methods would not be necessary. The rest of the API looks perfect for the tests we want to write.