Prevent merging commits that contain `it.only` in tests

[make-github-pseudonymous-again]

One example implementation at https://www.npmjs.com/package/eslint-plugin-no-only-tests. Ideally we would want this to run only in CI, no need to prevent PRs from containing such code BEFORE being merged. While most occurrences should be caught by coverage thresholds, better safe than sorry.

See also:

• https://github.com/cypress-io/eslint-plugin-cypress/issues/28
• https://github.com/vercel/style-guide/issues/9