Investigate the possibility of testing elements

[djowel]

Testing GUIs is tricky. but nevertheless, I don't feel good without tests. Right now, I have to build the examples and inspect each one of them to make sure everything is good.

At the very least, a quick testing facility is probably something like:

1. Saving the view to a PNG
2. Comparing the PNGs to known 'golden' files and reporting the diffs.

It might be possible to simulate user interaction, but that can probably be done later.

[Xeverous]

You might want to expose an interface that allows issuing events programatically. Eg issue a virtual click at some coordinates and verify that state changed accordingly. If rendering in well-separated from the logic it should be trivial to test correct behavior.

[djowel]

Done for Elements 1.0: golden-image comparisons plus simulated input, on every host and on a headless one, with CI on macOS, Linux and Windows: https://cycfi.github.io/elements/elements/v1.0/testing.html