Make sure we have testing for all proto buffer interfaces

[drrt]

From @kunzimariano on August 7, 2017 21:11

NodeAgent
Plugins

• Workload Attester
• Key Manager
• Node Attester

API

• Workload API

Copied from original issue: spiffe/node-agent#36

[walmav]

Decided to close it and make sure our test coverage handles this.