Use a non-cloud type for nested unit test

[liggitt]

It looks like #170 added a reference to google.golang.org/genproto/googleapis/longrunning which is an alias of cloud types. Is there an alternative non-cloud type that could be used for the "nested" test case at https://github.com/googleapis/gax-go/blob/main/v2/proto_json_stream_test.go#L78-L82?

Context:

• We're trying to isolate and simplify dependencies around the genproto and cloud.google.com libraries as part of Untangle genproto / cloud.google.com/go dependencies kubernetes/kubernetes#113366
• A unit test in this repo is referencing proto messages in packages which pull in cloud.google.com/go types and seem to accidentally expand the dependency graph significantly.
• chore(v2): switch ProtoJSONStream test to use non-cloud API #237 already dropped one of those types.

cc @quartzmo @noahdietz