proto: add Merge tests for aberrant inputs
proto: add Merge tests for aberrant inputs

The generated Go code does not perfectly represent the protobuf data model.
As such, it is possible to construct Go message values that could not
otherwise be constructed through Unmarshal or Merge.
This CL adds a test to exercise some of the boundary conditions.
The tests do not necessarily lock in the current behavior, but provides
a way for us to know when they have changed (either intentionally or not).

Change-Id: I389ef97c2fcf037595ad553baf5270d7bee9673b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/196738
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go