OMCI-lib-go: Test Result should support non-zero TID values

Most async notifications in OMCI always have a TID value of zero.  For TestResults, this is only true if the result was self-generated by the ONU. 

If the TestResults is due to a previous TestRequest (as in Girish Gowdra ANI-G test), it should be the value of the request.

So the non-zero TID check in the serializer and decoder for Test Result should be relaxed.