[test] Opt out of screenshot for flaky tests

[mj12albert]

Suggested in #39711 (comment)

• 1. Implement the NoSnap convention like MUI X https://github.com/mui/mui-x/blob/612fb24d6686565dd1adac613c2d42aadf3047d6/test/regressions/index.js#L19

const blacklist = [
  /^docs-(.*)(?<=NoSnap)\.png$/, // Excludes demos that we don't want
  // 'docs-system-typography',
];

• 2. Update the blacklist to use the prefix for demos.

[oliviertassinari]

@mj12albert Great, thanks for creating this issue 👌