fix(tests): Track visual regressions

[adrienjoly]

Refs:

• "applitools eyes" is visual snapshot service for Cypress
• I picked it from https://docs.cypress.io/plugins/#visual-testing because it was verified by Cypress and they provide a free plan for open source projects
• install steps: https://applitools.com/tutorials/cypress.html

--> https://eyes.applitools.com/app

[adrienjoly]

Problems I'm facing with the solution proposed in this PR:

1. I did not find a way to navigate back to the homepage, from the /login page, in Cypress (cf https://github.com/openwhyd/openwhyd/pull/413/files#diff-3893d76fb76135516a86c6fda5b739fc53f17633ea2fdd6861442f445ef9157bR45)
2. Applitools Eyes sometimes takes a snapshot too early, before the new content was injected into the body of the page (e.g. https://eyes.applitools.com/app/test-results/00000251795643670265/00000251795643670138/steps/4?accountId=zf2VcF6gtk2wqY43MNPslw~~&mode=step-editor)

=> About the second problem, here's what I posted on Twitter:

Is it me, or there is no Visual Regression testing plugin for Cypress that retries/wait until the screen matches the expected/baseline snapshot?
This makes it hard to implement visual testing on http://openwhyd.org, because pages are rendered programmatically when navigating
I.e. on http://openwhyd.org, whenever the user clicks on a link, some client-side JavaScript displays a loading animation, fetches the content from the server, injects it into the and then hides the loading animation.
So far, the only solution I found is to ask Cypress to check for the presence of some text on the page, after navigating... Visage avec des sueurs froides I find this solution too verbose and fragile.
=> @applitools, do you have any suggestion?
The test suite in question: https://github.com/openwhyd/openwhyd/pull/413/files#diff-3893d76fb76135516a86c6fda5b739fc53f17633ea2fdd6861442f445ef9157bR28

EDIT, for reference: Cypress automatically re-evaluates / retries assertions until they are true (cf https://docs.cypress.io/guides/references/best-practices.html#Unnecessary-wait-for-cy-get) => it would be awesome if Applitools could apply that same philosophy.