sdks/cpp needs unit tests

[devjgm]

The C++ SDK needs some unit tests to show that it works, and to allow people to refactor the code with (some) confidence it won't be broken. The standard unit testing framework that I'd suggest is https://github.com/google/googletest

[dsazonoff]

I'm working on it now in scope of #134.

[roberthbailey]

This is a dupe of #905 and since that one has more details, I'm going to close this one and use it as the canonical issue.