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.

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.

Fix nasa#1287, split interface and implementation modules

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

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

Fix nasa#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.

cFE Integration candidate: 2021-04-20