Improve documentation for unit tests fitness function rule

If someone needs to add tests to a file that already uses sinon, their build will fail with an error.

Currently, we point the developer to [generic documentation](https://github.com/MetaMask/metamask-extension/blob/develop/development/fitness-functions/rules/index.ts#L9) about unit tests when this rule fails.

We should point the user instead to the options he has for dealing with rule violations, namely:

a) refactor the existing test file as part of the same PR where they are trying to add new test cases or,
b) create a new test file for the new test cases without changing any of the existing cases

Start with development/fitness-functions/rules/index.ts at the linked unit-test documentation reference. Review how the rule reports violations, then update the guidance to describe refactoring the existing test file or creating a new test file; done means the failure points to these options instead of the generic documentation.

Written by the indexing model from the issue text.