Handle test resources gracefully from CMake Script

[zbeekman]

Some tests may require a minimum number of images to run correctly, however, on some systems oversubscribing seems to be broken with OpenMPI (although, I may be a knuckle head and #265 may be the cause of this... TBD). Either way, the macro for setting up the tests should allow an argument for requested images and minimum number of images and then disable tests if the system cannot provide the number of cores required. This will be a good feature to have either way...