Add tiny test for ToPickle (dask#6021)

distributed/protocol/tests/test_to_pickle.py

@@ -4,10 +4,22 @@
 from dask.highlevelgraph import HighLevelGraph, MaterializedLayer
 
 from distributed.client import Client
+from distributed.protocol import dumps, loads
 from distributed.protocol.serialize import ToPickle
 from distributed.utils_test import gen_cluster
 
 
+def test_ToPickle():
+    class Foo:
+        def __init__(self, data):
+            self.data = data
+
+    msg = {"x": ToPickle(Foo(123))}
+    frames = dumps(msg)
+    out = loads(frames)
+    assert out["x"].data == 123
+
+
 class NonMsgPackSerializableLayer(MaterializedLayer):
     """Layer that uses non-msgpack-serializable data"""