#217: Add tests for QueryFilter abstract class

- Similar testing principle to testing the abstract backend class

test/test_query_filter.py

@@ -0,0 +1,18 @@
+from datagateway_api.common.filters import QueryFilter
+
+
+class TestQueryFilter:
+    def test_abstract_class(self):
+        """Test the `QueryFilter` class has all required abstract methods"""
+
+        QueryFilter.__abstractmethods__ = set()
+
+        class DummyQueryFilter(QueryFilter):
+            pass
+
+        qf = DummyQueryFilter()
+
+        apply_filter = "apply_filter"
+
+        assert qf.precedence is None
+        assert qf.apply_filter(apply_filter) is None