WIP run test buffer in single fullscreen window and restore after finished

This is just an temporary solution for running test buffers as fullscreen windows.

* Select specific test buffer and show as single opened window.
* define `q` key inside of buffer to quite and restore last window state.

@senny What do you think about these kind of test flow? Consider that the code is really prototypal, so we could talk about how to do it later. :smile: