This reproduces a bug in AVA (maybe?). Just run $ npm t and you'll …

[kentcdodds]

…get:

(node) warning: possible EventEmitter memory leak detected. 11 test listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at TestCollection.addListener (events.js:239:17)
    at Runner.run (/Users/kdodds/Developer/eslint-find-new-rules/node_modules/ava/lib/runner.js:127:13)
    at process.<anonymous> (/Users/kdodds/Developer/eslint-find-new-rules/node_modules/ava/index.js:90:10)
    at emitOne (events.js:77:13)
    at process.emit (events.js:169:7)
    at process.<anonymous> (/Users/kdodds/Developer/eslint-find-new-rules/node_modules/ava/lib/test-worker.js:124:10)
    at emitTwo (events.js:87:13)
    at process.emit (events.js:172:7)
    at handleMessage (internal/child_process.js:689:10)
    at Pipe.channel.onread (internal/child_process.js:440:11)

[ta2edchimp]

According to the hint to use proxyquireto mock fs, replacing the mock-fs setup with the following seems to solve the issue:

npm i -D proxyquire

import proxyquire from 'proxyquire'
import test from 'ava'

const findNewRules = proxyquire('../index', {
  fs: {
    readdirSync: () => ['foo-rule.js', 'bar-rule.js', 'baz-thing.js'],
  },
})

(Of course, findNewRules could also be assigned using test.before.)

Edit: it solved the issue with the original tests.
Edit 2: I altered the example above to work with the tests from this PR. Sorry for adding so much noise.

[jamestalmage]

Cool - this solves it even easier than I was thinking.

I was actually pointing to the mock.fs(..) method of the mock-fs module. It creates a fs clone without clobbering the native one.

Just an FYI - mount-fs looks like a way smarter way to use mock-fs. Use the mock.fs(..) method to create a clone, and mount it in a specific directory using mount-fs.