Ensure that wxDataViewMainWindow has correct size in the test

Replace a redundant (because the same size was already specified in the
ctor) SetSize() call with a Layout() call which resizes
wxDataViewMainWindow to fit the parent control size when using the
generic implementation.

This is important for any tests dealing with the control geometry, i.e.
calling GetItemRect() or HitTest().

tests/controls/dataviewctrltest.cpp

@@ -83,7 +83,7 @@ DataViewCtrlTestCase::DataViewCtrlTestCase(long style)
         m_grandchild = m_dvc->AppendItem(m_child1, "grandchild");
       m_child2 = m_dvc->AppendItem(m_root, "child2");
 
-    m_dvc->SetSize(400, 200);
+    m_dvc->Layout();
     m_dvc->Expand(m_root);
     m_dvc->Refresh();
     m_dvc->Update();