Document nested usage of UT_ADD_TEST and clarify nomenclature for individual versus groups of tests

[astrogeco]

Describe the problem

The UT_ADD_TEST call has some limitations as addressed in #840. Some solutions are discussed in that PR.

Proposed Solutions

1. Have UT_ADD_TEST handle "nested" unit tests, OR
2. Develop a nomenclature for unit test functions that explicitly indicates whether there are nested unit tests and then rename unit tests accordingly, OR
3. Document the anti-pattern, OR
4. Split up all unit tests to ensure only one test per function and make this a design pattern.

Additional Context

Originally posted by @skliper in #840 (comment)

Related to nasa/osal#577. Individual tests are added w/ the UT_ADD_TEST call. Some tests are grouped at a lower level (Test_MSG_CCSDSExt is a group of tests), and those functions are called directly. Same pattern is used in sb:

cFE/fsw/cfe-core/unit-test/sb_UT.c

Lines 134 to 135 in a148b97

	Test_SendMsg_API();
	UtTest_Add(Test_RcvMsg_API, NULL, Test_CleanupApp_API, "Test_RcvMsg_API");

[skliper]

Documented in nasa/osal#577 and addressed by nasa/osal#751, closing here.