[test] Run snapshot tests on the a11y tree

[eps1lon]

The actual logic is done in https://github.com/eps1lon/mui-scripts-incubator/tree/master/lib/a11y-snapshot.

It saves a snapshot of the a11y tree for https://material-ui.netlify.com/ and https://material-ui.netlify.com/components/select and checks these against new pushes on master as well as PRs.

example snapshot

<WebArea>
  "Material-UI: A popular React UI framework"
  <link>
    "Skip to content"
  </link>
  <button>
    "Open main navigation"
  </button>
  <combobox autocomplete="list" haspopup="listbox">
    "Search"
  </combobox>
  <combobox description="Change language" haspopup="menu">
    "Change language"
  </combobox>
  <link description="Edit website colors">
    "Edit website colors"
  </link>
  <button description="Toggle light/dark theme">
    "Toggle light/dark theme"
  </button>
  <button description="Toggle right-to-left/left-to-right">
    "Toggle right-to-left/left-to-right"
  </button>
  <combobox description="Toggle notifications panel" haspopup="menu">
    "Toggle notifications panel"
  </combobox>
  <link description="GitHub repository">
    "GitHub repository"
  </link>
  <main>
    ""
    <heading level="1">
      "MATERIAL-UI"
    </heading>
    <heading level="2">
      "React components for faster and easier web development. Build your own
      design system, or start with Material Design."
    </heading>
    <link>
      "GET STARTED"
    </link>
    <link>
      "Star"
    </link>
    <Iframe>
      "Twitter Follow Button"
    </Iframe>
    <link>
      "Get Professional Support"
    </link>
    <text>
      "A quick word from our sponsors:"
    </text>
    <link>
      "a random quick word"
    </link>
    <heading level="3">
      "Installation"
    </heading>
    <text>
      "Install Material-UI's source files via npm. We take care of
      injecting the CSS needed.$ npm install @material-ui/core"
    </text>
    <link>
      "or use a CDN."
    </link>
    <text>
      "Load the default Roboto
      font.<linkrel="stylesheet"href="https://fonts.googleapis.com/css?family=Roboto:300,400,500,700&display=swap"/>"
    </text>
    <link>
      "READ INSTALLATION DOCS"
    </link>
    <heading level="3">
      "Usage"
    </heading>
    <text>
      "Material-UI components work without any additional setup, and don't
      pollute the global
      scope.importReactfrom'react';importButtonfrom'@material-ui/core/Button';constApp=()=>(<Buttonvariant="contained"color="primary">Hello
      World</Button>);"
    </text>
    <link>
      "EXPLORE THE DOCS"
    </link>
    <heading level="2">
      "Premium themes"
    </heading>
    <text>
      "Take your project to the next level with premium themes from our store –
      all built on Material-UI."
    </text>
    <link>
      "Browse themes"
    </link>
    <link>
      "BROWSE THEMES"
    </link>
    <heading level="2">
      "Material-UI's sponsors"
    </heading>
    <text>
      "The continued development and maintenance of Material-UI is made possible
      by these generous sponsors:"
    </text>
    <heading level="3">
      "Diamond 💎"
    </heading>
    <text>
      "3/3 slots available"
    </text>
    <heading level="3">
      "Gold 🏆"
    </heading>
    <link>
      "tidelift"
    </link>
    <link>
      "bitsrc"
    </link>
    <link>
      "call-em-all"
    </link>
    <heading level="3">
      "There are more!"
    </heading>
    <text>
      "See the full list of"
    </text>
    <link>
      "our sponsors"
    </link>
    <text>
      ", and learn how you can contribute to the future of Material-UI."
    </text>
    <heading level="2">
      "Praise for Material-UI"
    </heading>
    <text>
      "a random quote about Material-UI"
    </text>
    <link>
      "a random quote about Material-UI"
    </link>
    <link>
      "a random quote about Material-UI"
    </link>
    <link>
      "a random quote about Material-UI"
    </link>
    <heading level="2">
      "a random quote about Material-UI"
    </heading>
    <text>
      "Join these and other great organisations!"
    </text>
    <img>
      "NASA"
    </img>
    <img>
      "Walmart Labs"
    </img>
    <img>
      "Capgemini"
    </img>
    <img>
      "Uniqlo"
    </img>
    <img>
      "Bethesda"
    </img>
    <img>
      "J.P. Morgan"
    </img>
    <img>
      "Shutterstock"
    </img>
    <img>
      "Netflix"
    </img>
    <img>
      "Coursera"
    </img>
    <img>
      "Amazon"
    </img>
    <img>
      "Unity"
    </img>
    <text>
      "Are you using Material-UI?"
    </text>
    <link>
      "LET US KNOW!"
    </link>
  </main>
  <link>
    "Material-UI"
  </link>
  <heading level="2">
    "Community"
  </heading>
  <link>
    "GitHub"
  </link>
  <link>
    "Twitter"
  </link>
  <link>
    "StackOverflow"
  </link>
  <link>
    "Team"
  </link>
  <heading level="2">
    "Resources"
  </heading>
  <link>
    "Support"
  </link>
  <link>
    "Blog"
  </link>
  <link>
    "Material Icons"
  </link>
  <heading level="2">
    "Company"
  </heading>
  <link>
    "About"
  </link>
  <link>
    "Contact Us"
  </link>
  <link>
    "Jobs"
  </link>
  <text>
    "Currently"
  </text>
  <link>
    "v4.9.5. View versions page."
  </link>
  <text>
    ". Released under the"
  </text>
  <link>
    "MIT License"
  </link>
  <text>
    ".Copyright ©2020Material-UI."
  </text>
</WebArea>

Since it's not clear how useful that information is I moved the logic into a separate behavior. This means that the PR workflow in this repository is unaffected. We don't want to burden contributors with experimental workflows and false alarms (see argos and test_browser).

[mui-pr-bot]

No bundle size changes comparing 440a68f...789ef79

Generated by 🚫 dangerJS against 789ef79

[eps1lon]

Tested in https://github.com/eps1lon/material-ui-playroom/runs/492647605?check_suite_focus=true

[eps1lon]

Communication is working: https://github.com/eps1lon/mui-scripts-incubator/actions/runs/51451071

We just need to move the trigger so that the message is sent after a successful deploy not right after the push. Need to push this to master in order to get a workflow run on status.