Add backdrop api and test

test_tjnetwork.py

@@ -2488,5 +2488,44 @@ class TestApi:
         self.leave(p)
 
 
+    # 27 backdrop
+
+
+    def test_backdrop(self):
+        p = 'test_backdrop'
+        self.enter(p)
+
+        assert get_backdrop(p)['content'] == ''
+
+        set_backdrop(p, ChangeSet({'content': 'x'}))
+        assert get_backdrop(p)['content'] == 'x'
+
+        self.leave(p)
+
+
+    def test_backdrop_op(self):
+        p = 'test_backdrop_op'
+        self.enter(p)
+
+        cs = set_backdrop(p, ChangeSet({'content': 'x'})).operations[0]
+        assert cs['operation'] == API_UPDATE
+        assert cs['type'] == 'backdrop'
+        assert cs['content'] == 'x'
+
+        cs = execute_undo(p).operations[0]
+        assert cs['operation'] == API_UPDATE
+        assert cs['type'] == 'backdrop'
+        assert cs['content'] == ''
+
+        cs = execute_redo(p).operations[0]
+        assert cs['operation'] == API_UPDATE
+        assert cs['type'] == 'backdrop'
+        assert cs['content'] == 'x'
+
+        self.leave(p)
+
+
+
+
 if __name__ == '__main__':
     pytest.main()