Unit test won't build with external time source configuration

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Is your feature request related to a problem? Please describe.
Many failures in build with ENABLE_UNIT_TESTS=true and the following settings:

#define CFE_PLATFORM_TIME_CFG_SOURCE true
#define CFE_PLATFORM_TIME_CFG_SRC_TIME true
#define CFE_MISSION_TIME_CFG_FAKE_TONE false

This has already been noted in the general time unit testing issue #109 which includes many other unit test issues with changing time config. Raising priority on this one with a unique issue since it's currently a stakeholder impact.

Describe the solution you'd like
Fix this configuration at minimum, which is often used when supplying cFS with time from an external source (or app).

Describe alternatives you've considered
Time services does need a general refactor as noted in #302, many issues w/ the unit tests as noted in #109.
Really would benefit from converting the configuration files to cmake as described in #2089.

Additional context
#2072 fixed the build issue, but unit test fails to build.

Requester Info
Jacob Hageman - NASA/GSFC per stakeholder request

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Ping @jonathan-brandenburg-metecs for awareness.