handle django 2.2 not excluding test via db feature flag

Author: Tim Nyborg

testapp/settings.py

@@ -56,6 +56,7 @@
                   'expressions_case.tests.CaseExpressionTests.test_annotate_with_in_clause',
                   'expressions_window.tests.WindowFunctionTests.test_nth_returns_null',
                   'expressions_window.tests.WindowFunctionTests.test_nthvalue',
+                  'expressions_window.tests.WindowFunctionTests.test_range_n_preceding_and_following',
                   'ordering.tests.OrderingTests.test_orders_nulls_first_on_filtered_subquery',
                   'queries.test_bulk_update.BulkUpdateNoteTests.test_set_field_to_null',
                   'get_or_create.tests.UpdateOrCreateTransactionTests.test_creation_in_transaction',