Fix wxSearchCtrl::HasFocus() in the generic version.

Fix wxSearchCtrl::HasFocus() in the generic version.

Override GetMainWindowOfCompositeControl() in the windows composing the
generic wxSearchCtrl to ensure that HasFocus() correctly returns true for the
entire control when any of them has focus.

Add a trivial unit test to check that this really works and facilitate adding
more tests for this class later.

Closes #14970.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73401 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775