Add msg module stubs, update SB to use them, and no longer include actual msg target in unit test builds

[skliper]

Is your feature request related to a problem? Please describe.
Follow on to #726, since it leaves the msg module as "internal" in the context of unit tests (it includes the target). This was done such that it could be shown all the old SB unit tests would pass (w/ minor updates for the bug fixes and reorg).

Describe the solution you'd like
Add the stubs, rework the SB unit tests, and remove the msg target from UT builds.

Describe alternatives you've considered
N/A

Additional context
#726

Requester Info
Jacob Hageman - NASA/GSFC

[skliper]

@excaliburtb ping.