Extend the msgpack unit tests to cover all decoding and encoding execution paths

### Subject of the issue
Extend the msgpack unit tests to cover all decoding and encoding execution paths

### Details
Our existing msgp decoders and encoders are auto-generated by the msgp utility.
This utility generates not only the decoder and encoder code, but also auto-generate unit testing that cover the above functionality.
We need to extend this unit testing to cover all the generated code. As a verification method, we can use the code coverage to see that the entire msgp_gen.go file is being covered.

### Next Steps
- Need to check what test cases are not covered to see if this is worth doing