Documenting and fixing ADDED_XXX output arguments (TriBITS #4)

I changed the name of the output arg to ADDED_EXE_TARGET_NAME since "exe" was
more common than "exec" in the these functions.

The implementation of the output argument ADDED_TESTS_NAMES_OUT for
TRIBITS_ADD_TEST() did not work at all.  It is now fixed and tested.

The implementation of the arguments ADDED_EXE_TARGET_NAME_OUT and
ADDED_TEST_NAMES_OUT for TRIBITS_ADD_EXECUTABLE_AND_TEST() did not work at
all.  They are now fixed and tested.

Nothing was documented or tested.  Now everything is documented and tested.

However, note that I don't have automated testing for TRIBITS_ADD_LIBRARY(),
TRIBITS_ADD_EXECUTABLE(), or TRIBITS_ADD_EXECUTABLE_AND_TEST().  So to test
that the output arugmments work, I put in some verbose-mode print statements
and then grep these.  I think such print statements would be pretty common in
real CMakeLists.txt files but it does clutter up the files some.

Fixes #4.