Add tests...

[ErikReider]

This would really speed up development due to a reduction in manually testing everything before pushing a new feature :)

Tests that I can think of. Post a comment if I missed one.

• [ ] All swaync-client args
• [ ] All subscription actions output a correct swaync-client subscription value
• [ ] Blank windows being shown and not opened on the focused monitor
• [ ] Notification actions
• [ ] DND with different notification urgencies
• [ ] Scripts
• [ ] Keyboard shortcuts (if this even is possible...)
• [ ] Styling if this even is possible (maybe screenshots with manual approval???☹️)
• [ ] Verify notifications moving from NotificationWindow to the Control Center when cc is opened
• [ ] Notification icon priority (provided -> app icon -> desktop entry icon -> default icon)
• [ ] Notification icon visibility depending on config setting
• [ ] Body markdown
• [ ] Body image
• [ ] CC margin and positioning
• [ ] Notification timeouts
• [ ] Notification visibility
• [ ] Config and CSS expand paths (start with "~/", "/" or "./")
• [ ] Config and CSS loading
• [ ] Config values