Initial version of snapshot testing

[spirali]

Hi,

This PR contains an initial version of snapshot testing.

By design it contains only a minimal API and usage inspired by vello snapshot tests.
The ambition of this PR is not to do any test coverage, it contains only two tests for demonstration purposes of the testing environment.

All the bells and whistles (html report, image diffs, not loading fonts in every test) are now missing to make the code more straightforward.

Usage

$ cargo test

If a test fails, you can compare images in parley/tests/current (images created by the current test)
and parley/tests/snapshots (the accepted versions).

If you think that everything is ok, you can start tests as follows:

$ PARLEY_TEST="accept" cargo test

It will update snapshots of the failed tests.

How it works

It reuses tiny-skia-renderer and renders the testing layout into an image and compares it pixel by pixel.

This PR also adds fonts into the tests, to have tests independent on the system fonts. I have chosen DejaVu fonts, but it was a random pick.

[DJMcNab]

Thank you - it will be great to get some testing landed.

However, We definitely shouldn't import several megabytes of fonts - it's pretty much a worst-case scenario for git; I'd suggest using only two fonts files; presumably we do want to test multi-font cases.
We also need to follow the license for whatever fonts we do import.

[spirali]

We definitely shouldn't import several megabytes of fonts.

We also need to follow the license for whatever fonts we do import.

The fonts are used only in tests, Parley itself can be build without it.

In the last change I have added license of the fonts.

[spirali]

Thank you - it will be great to get some testing landed.

However, We definitely shouldn't import several megabytes of fonts - it's pretty much a worst-case scenario for git; I'd suggest using only two fonts files; presumably we do want to test multi-font cases. We also need to follow the license for whatever fonts we do import.

Ok, I will try to prune it. However it would not be that bad for git, these files should only rarely changed, so it just a few constant megabytes.

I have also attached more variants so we can test e.g. different weights, because variable weighted font does not for me in Parley (I do not know if it a feature or a bug).

[spirali]

I have removed (and squashed) other variants and left just DejaVu Sans and DejaVu Sans Bold (= 2x 700KiB)

[xorgy]

It might make more sense to use a pinned version of a distro font package in the eventual CI (maybe Noto?), and just have it spit out a useful message when you don't have those fonts installed.

[spirali]

I have completely removed fonts from repository & added a script that downloads Noto Fonts.

[spirali]

Note: CI is now failing, because font download is not part of CI scripts.