Add a minimal unit test for wxSimplebook.

This is just a test running the common wxBookCtrlBase tests for wxSimplebook.

See #15188.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74277 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
No real changes, just simplify wxBookCtrlBase unit test a little.

Add Realize() virtual function that can be overridden in ToolbookTestCase
instead of using explicit wxDynamicCasts.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74278 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Add wxBookCtrlBase::DoSetSelectionAfterRemoval() and use it in wxSimplebook.

This fixes the wrong handling of the selection in wxSimplebook when the
currently selected page was deleted.

Also extend the unit tests to check for this bug.

Closes #15188.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74279 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775