CMake: automatize testing

[fkobi]

Right now if an autotools user wants to test the compiled code they need to do that manually.

Since the process described in the readme is repetetive this could be achieved by the build system (ex. ENABLE_TESTING option).

Another added benefit would be that the test programs needing the tiff library could not be built.
That way a build with -DWITH_TIFF=OFF and -DENABLE_TESTING=OFF would not need libtiff all, as it now does.