Fix drawing of expander columns not at 0 position in generic wxDVC.

Fix drawing of expander columns not at 0 position in generic wxDVC.

The drawing code in the generic version of wxDataViewCtrl incorrectly supposed
that the expander column was always at position 0. This resulted in the
expander column not being drawn at all if it was not really the first one.

Fix the test to use wxDataViewCtrl::GetExpanderColumn() to correct this.

Closes #12870.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68321 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775