Keep expand/collapse state of tree nodes when changing the tree filter

[rowo360]

This PR is another contribution to the test filter functionality (#1148, #1161, #1147).

This PR propose to keep the expand/collapse state of the tree nodes constant, whenever a test filter is modified. For example if a user expanded certain tree nodes and afterwards apply a test filter, all those tree nodes which are still visible after the filtering should remain their expand/collapse state.

The idea is that the user is familiar with the view after the filtering and does not have to adjust the expand/collapse state again right away. So overal this PR tries to improve the usability when working with test filters.

[rowo360]

I have the impression that I don't have to invent anything new for this functionality. Instead the expand/collapse state is already handled by the VisualState file, so I just want to apply it in this use case.

Whenever a test filter is modified, the DisplayStrategy.Reload() method is invoked. This method in turn calls the OnTestLoaded(TestNode, VisualState) method. So far the second parameter was set to null, which means that some default handling for expanding/collapsing tree nodes is applied.
Now I determine the VisualState first and pass it to the OnTestLoaded method - that's all to keep the expand/collapse state constant.

[rowo360]

Just for the sake of completeness here are two screenshots showing the existing code when no VisualState is passed to the OnTestLoaded() method: