Add a11y testing to pixel

[bernardwang]

Summary

Adds ability to test for a11y errors to pixel.

• Add new a11y-regression docker image
• Use the pa11y library to run automated a11y tests
• Create a custom a11y error reporter that shows differences between the 'test' and 'reference' patches

Notes

At some point, it might make sense to factor out the part of pixel that creates docker containers to compare patches. That logic is what's primarily being leveraged for this new feature, and having a generic tool for comparing patches could be useful for other applications as well.

How to test

The -a or --a11y flag is used to run a11y tests instead of visual tests.

Run the following to see what a a11y regression would look like

./pixel.js reference -a && ./pixel.js test -c 954075 -a

Run the following to see what a a11y improvement would look like

./pixel.js reference -c 954075 -a && ./pixel.js test -a

Run the following to see what no change would look like

./pixel.js reference -a && ./pixel.js test -a

[montehurd]

Hey @bernardwang

Sorry for delay! Seems to run fine on my machine

Could the Dockerfile be updated to use this?

It's not a dealbreaker, just curious.

[bernardwang]

@montehurd done!