Clean up `TestAgent` implementation

[turtlemonvh]

We're close to a mock the functionality I want for testing, but I'd like a bit more.

• Implement updateKeys and updateKey.
• Fix getKeys to properly handle setting up mappedIdList.
• Provide some tools for fault injection.
  • Start with faking out loss of network connectivity and server side errors.
• Clean up the use of refId
• Clean up use of error codes for missing key id / missing external id
• Ensure we add the right items to the GetKeysResponse for each key query (IonicError, QueryResult, and Key)

[turtlemonvh]

Progress

• I checked out out the internal implementation of GetKeysResponse and I think I have the scenarios for IonicError, QueryResult and Key worked out correctly.
• I fixed the mappedIdList handling in getKeys.
• Error scenarios for missing key ids and external ids seems to be handled correctly (added tests for this too)

Still to do

• Implement updateKeys and updateKey.
• Provide some tools for fault injection.
• Clean up the use of refId

[turtlemonvh]

Progress

• Implemented updateKeys and updateKey.
• Provided some tools for fault injection.

I decided the tooling around refId doesn't need to be changed now. I can add to that later if needed.