Most parts of ESLint have unit tests associated with them. Unit tests are written using Mocha and are required when making contributions to ESLint. You'll find all of the unit tests in the tests directory.
When you first get the source code, you need to run npm install once initially to set ESLint for development. Once you've done that, you can run the tests via:
npm test
This automatically starts Mocha and runs all tests in the tests directory. You need only add yours and it will automatically be picked up when running tests.
If you want to quickly run just one test, you can do so by running Mocha directly and passing in the filename. For example:
npm test:cli tests/lib/rules/no-wrap-func.js
Running individual tests is useful when you're working on a specific bug and iterating on the solution. You should be sure to run npm test before submitting a pull request.
npm test:cli is an alias of the Mocha cli in ./node_modules/.bin/mocha. Options are available to be provided to help to better control the test to run.