Add proper testing

[serenity4]

The package currently fails for CI testing, because the tests are made to be interactively run: opening a window, pressing some keys, moving the mouse around, closing the window.

Testing can be done with a virtual framebuffer through xvfb. However, we need to manually send events to the X server, since no input device can be used. A solution consists in sending events to the X server with xcb_send_event, thus faking inputs. To do that, we need a way to translate some keystroke (let's say, I want to press 'a') to a keycode, that is used as event data.

[serenity4]

This is a good opportunity to actually work on properly handling inputs. For now, there is no keymap registration, and keyboard state is not considered for converting keycodes to characters. xkbcommon and xcb-xkb handle this kind of stuff, including weird keyboard layouts.

[serenity4]

Related PR: #6

[serenity4]

Tests now ensure that windows can be created, receive events, and close upon an emulated keystroke. The issue is therefore resolved.