Add unit test example for sample_lib

[jphickey]

Is your feature request related to a problem? Please describe.
The SAMPLE library does not implement any unit test. Actual CFE libraries would typically need to do this.

Describe the solution you'd like
A unit test should be implemented to provide a template for implementing a library unit test (code coverage) using the current framework.

Additional context
The unit test for an application (e.g. in sample_app, as in nasa/sample_app#21) is slightly different, as the application typically would not need to expose stubs but a library would. Therefore both sample_app and sample_lib should implement unit tests.

Requester Info
Joseph Hickey, Vantage Systems, Inc.