[FAB-3461] Add UT for gossip protobuf extensions.

This commit adds unit tests to cover gossip message types validatiod and
ensures correct usage of gossip tagging policies.

Change-Id: I86bc6d9d5a6ed28436dfb78f1377394f2e8d3401
Signed-off-by: Artem Barger <[email protected]>

Author: C0rWin