frontend/mina-signer: add 'jest' dev-dependency to run tests in CI

[yorickvP]

Explain your changes:

• I'd like to run npm test in CI, which depends on jest. This commit adds the jest executable to the dev dependencies of mina-signer
  Explain how you tested your changes:
• npm test

Checklist:

• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them