Add lint to remind you not to leave an focused/exclusive test in

[MadLittleMods]

Relevant rule: https://github.com/jest-community/eslint-plugin-jest/blob/ad04fcc97b5dcc676c5bd2152f84608c08e0c50a/docs/rules/no-focused-tests.md

Spawning from #2856 (comment)

Alternatives

There are other ways we could add this functionality but this seems most relevant to us. https://timdeschryver.dev/blog/dont-commit-focused-tests has a decent summary.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[t3chguy]

This should live in https://github.com/matrix-org/eslint-plugin-matrix-org

[MadLittleMods]

Moved to matrix-org/eslint-plugin-matrix-org#33