Integration testing

We've created unit tests for our Mock Battery, but we haven't tested it in situ for its behaviors yet.

Although we do have console output and a main() function available we can run to see some of this, we haven't created a similar test for this.

We will revisit testing -- and specifically integration testing -- after we have completed the exercise for the Charger component. Once that is available, we'll be able to test a combined integration that validates the behavior of our battery as it is discharged and charged through different cycles.