[Cmake] ENABLE_TESTING and ADD_TEST

[Brad King]

> I think ADD_TEST and ENABLE_TESTING should work hand-in-hand: if
> ENABLE_TESTING has not been run, then ADD_TEST sliently does nothing.
> 
> Another option is to remove the ENABLE_TESTING command altogether,
> making it always on. The possibility of tests will then always be
> available.
This was discussed when we originally designed the ENABLE_TESTING
command.  The intention was that ADD_TEST should not do anything unless
ENABLE_TESTING has been called.  The idea is that we don't want to have
Dart testing files show up in every build of every project ever using
CMake.  It is a bug that ADD_TEST is still executing even when
ENABLE_TESTING has not been invoked.  Any project that wants testing
should call ENABLE_TESTING in the topmost CMakeLists file, or in a file
that includes it.

-Brad