[Cmake] Added make test rule on UNIX

[Ken Martin]

I have updated cmake to build and install a program called ctest.
ctest will run the tests specified in the CMakeLists.txt files for a
CMake tree. I also modified cmUnixMakefileGenerator to add a test
target.  So theoretically you should be able to do something like

cd Insight-Linux
cmake ../Insight
make
make test

I have tested it a little bit with VTK but let me know if you run into
any problems.

Thanks
Ken