[CMake] How to specify working directory for unit tests?

[Tron Thomas]

I need to be able to run a certain unit from a specific directory so 
that it will be able to find some supporting file that are needed for 
the test.  However CTest insists on launching the test from a different 
location.

I tried using the --build-run-dir and --build-exe-dir flags.  Neither of 
them would change the unit test directory.

What can be done to specify the directory where CTest should launch the 
unit test?