Get tests running on Mac OS X.

On Mac OS X, running the tests in CTest fails currently because SFML links to some binary-blob frameworks it includes with its source (shudder). The path to these frameworks is specified using @rpath, which – since they are not installed in an rpath – means they are not found by default.

Setting the DYLD_FRAMEWORK_PATH environment variable to include the path to the frameworks causes the tests to pass. There is almost certainly a better way to do this.