Mock function A called within B when A is defined within the same file

I have a file (example.js) that looks like this and I'd like to test it.

function alpha () {
  return 'alpha'
}

function beta () {
  return alpha() + ' ' + 'beta'
}

Here's an example of the test for that file:

const test = require('ava')
const {beta} = require('./example')

test(t => {
  t.is(beta(), 'alpha beta')
})

I'm interested in having mocking the use of alpha within beta, when beta is defined within example.js. I know things that mock the require call so that dependencies could be mocked, but that's not what I'm looking for.