adapt tests

* Don't test removed accessor attribute
* Allow duplicate accessors

src/core/tests/widgets/test_table.py

@@ -135,24 +135,16 @@ def test_add_column(self):
 
         self.assertEqual(self.table.headings, expecting_headings)
 
-    def test_add_columns_accessor_in_use(self):
-
-        new_heading = 'Heading 4'
-        accessor = 'heading_2'
-
-        with self.assertRaises(ValueError):
-            self.table.add_column(new_heading, accessor)
-
     def test_remove_column_by_accessor(self):
         remove = 'heading_2'
         dummy_data = [
             ['a1', 'b1', 'c1'],
         ]
         self.table.data = dummy_data
 
-        expecting_accessors = [h for h in self.table._accessors if h != remove]
+        expected_columns = [col for col in self.table.columns if col.text != remove]
         self.table.remove_column(remove)
-        self.assertEqual(self.table._accessors, expecting_accessors)
+        self.assertEqual(self.table.columns, expected_columns)
 
     def test_remove_column_by_position(self):
         remove = 2