[CMake] compare two files - testing

[James C. Sutherland]

I have a test that produces output files that I would like to compare  
against a "blessed" copy.  Is there a way to do this in CMake/CTest?   
This is probably a very simple thing, but I have not figured out how  
to do it.

I have an
	add_test( ... )
that creates the test, but I am not sure how to add the diff on the  
output.

FYI, I am running this via "make test" - I am not yet running ctest  
directly.