added some tests

Added tests for `context_error` and `unsupported_extension`.
hmm, i'm having some trouble running this line, does it work for you? i think it's failing for me because there is no active OpenGL context to set up the OpenCL sharing context with. i think we first would have to make an OpenGL context (perhaps by creating a `QGLWidget` and displaying it). this is a bit more work (which is why i haven't gotten to it before), but it would be really good to start testing the OpenGL interop code.