Add functional tests for cFE External Time Source APIs

[zanzaben]

Is your feature request related to a problem? Please describe.
Need open source functional tests for certifiability

Describe the solution you'd like
Add functional tests for cFE External Time Source APIs
CFE_TIME_ExternalTone - Provides the 1 Hz signal from an external source.
CFE_TIME_ExternalMET - Provides the Mission Elapsed Time from an external source.
CFE_TIME_ExternalGPS - Provide the time from an external source that has data common to GPS receivers.
CFE_TIME_ExternalTime - Provide the time from an external source that measures time relative to a known epoch.
CFE_TIME_RegisterSynchCallback - Registers a callback function that is called whenever time synchronization occurs.
CFE_TIME_UnregisterSynchCallback - Unregisters a callback function that is called whenever time synchronization occurs.

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Additional context
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Requester Info
Alex Campbell GSFC