UDA Tester `Deserialize` tests merge; should test deserialize

[etep]

In file src/carnot/udf/test_utils.h, the UDF tester class provides a Deserialize method. That method invokes Merge instead of Deserialize. This is probably unintentional; it should, instead, test deserialize.

See: https://github.com/pixie-io/pixie/blob/d81a9fe3c3f1ab4163cfb1286accdf0d840573df/src/carnot/udf/test_utils.h#L202

Perhaps we should create a new method Merge and update the deserialize method to invoke deserialize instead.