Separate the list of CFE core interface modules (e.g. core_api) from
the list of CFE core implementation modules (e.g. msg).  This allows
the content of core_api to be expanded to include any additional
modules the user has added to cFE core locally.

Merge the "testrunner" app and put the runner logic into cfe_assert lib.
Converts the "testcase" library into an app that simply calls the logic
in cfe_assert to do the test.

This means that the test can be run on demand by issuing the ES
start app command, rather than putting it into the startup script.

Adjust the event types so that Start/End are "info" events, not debug
events (in the default platform config, debug events are not shown).

Also update documentation to correct the startup script entries.

Note in the documentation for this function that it is the responsibility
of the calling app to clear/fill the CDS block any time a new block is
allocated.

Use a constant at the top of the file, and change the
names to match current environment.

Fix #1287, split interface and implementation modules

Fix #1337, add docs to CFE_ES_RegisterCDS() regarding clearing

Fix #1281, remove testrunner and convert testcase to app

Building "cfe_assert" does not need to be conditional on
ENABLE_UNIT_TESTS, it should be built all the time.