Build system needs to better separate the UT stub function from the UT test cases

[jphickey]

Describe the bug
Currently the "unit-test" subdirectory generates libraries that provide stubs for all public API calls defined in the CFE core apps, as well as test cases and executables for unit testing the CFE itself.

Having these combined like this causes a few problems:

• the two products have different scopes. The UT stubs apply globally across the entire mission and can/should be used for all UT (platform-independent), but the test cases do need to be tuned to the platform, so a separate set of test cases needs to be built per-platform.
• As a result, the UT stubs themselves are also built per-platform. This is wasteful but more importantly this makes linking to the CFE stubs from application UT code difficult.
• It is also undesirable just from a general code organization standpoint. The directory structure would be cleaner if these were in separate subdirectories.

To Reproduce
Build code with ENABLE_UNIT_TESTS=TRUE and observe that the CFE "stub" library is named "libut_cfe_core_default_cpu1_stubs.a" (i.e. build specifically for the default/cpu1 platform config) and it resides in the same directory as the UT executables/test cases.

Expected behavior
The stub library should be named only "libut_cfe_core_stubs.a" and it should be in a separate subdirectory from the unit test cases themselves.

System observed on:
Ubuntu 18.04 LTS 64 bit

Reporter Info
Joseph Hickey, Vantage Systems, Inc.