UDA Tester `Deserialize` tests merge; should test deserialize

[Pete Stevenson]

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:

pixie/src/carnot/udf/test_utils.h

Line 202 in d81a9fe

 uda_.Merge(nullptr, other); 

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