docs(legacy): add test case to ensure correct csp hashes in readme.md

[KAROTT7]

Description

fix #13373

updated CSP hashes in README.md and added a test case to ensure correct.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[sapphi-red]

Thank you!
Would you move the test in packages/legacy/__tests__ since this test doesn't require the browser?

[KAROTT7]

Okay, I will do it in tomorrow’s morning

[sapphi-red]

LGTM

misc changes I pushed

• move the test case to a different file: this test is not related to snippets.ts
• use encoding option of fs.readFileSync instead of Buffer::toString: maybe it's a preference but I think it's more clear
• use expect(hashesInDoc).toStrictEqual(cspHashes) instead of expect(cspHashes.every).toBeTruthy(): this will give us a better failure message