Preserve the invariant tostring(elem) == b''.join(tostringlist(elem)) and add a test to make sure it keeps working

Lib/xml/etree/ElementTree.py

@@ -1190,9 +1190,15 @@ class DataStream(io.BufferedIOBase):
         def writable(self):
             return True
 
+        def seekable(self):
+            return True
+
         def write(self, b):
             data.append(b)
 
+        def tell(self):
+            return len(data)
+
     ElementTree(element).write(DataStream(), encoding, method=method)
     return data