Add an explicit SetFocus() call to fix wxTreeCtrl unit test.

Add an explicit SetFocus() call to fix wxTreeCtrl unit test.

Fixing the implicit focus grabbing by wxTreeCtrl::SelectItem() in r65905 broke
its unit test case as the simulated key event was not delivered to wxTreeCtrl
itself any more.

Fix this by simply setting the focus to the tree explicitly before sending it
any key strokes.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66170 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775