Fix the just added wxDataViewCtrl::DeleteAllItems() test

Using GetTopItem() was wrong, we actually need to use the invalid item,
which corresponds to the "virtual root" of wxDataViewTreeStore.

tests/controls/dataviewctrltest.cpp

@@ -266,12 +266,14 @@ TEST_CASE_METHOD(SingleSelectDataViewCtrlTestCase,
                  "wxDVC::DeleteAllItems",
                  "[wxDataViewCtrl][delete]")
 {
-    const wxDataViewItem top = m_dvc->GetTopItem();
-    CHECK( m_dvc->GetChildCount(top) == 1 );
+    // The invalid item corresponds to the root of tree store model, so it
+    // should have a single item (our m_root) initially.
+    CHECK( m_dvc->GetChildCount(wxDataViewItem()) == 1 );
 
     m_dvc->DeleteAllItems();
 
-    CHECK( m_dvc->GetChildCount(top) == 0 );
+    // And none at all after deleting all the items.
+    CHECK( m_dvc->GetChildCount(wxDataViewItem()) == 0 );
 }
 
 #endif //wxUSE_DATAVIEWCTRL