Mock computed property of Vue.js mixin

test.spec.js

import Vue from 'vue'
import 'babel-polyfill'
import Component from '~/components/Component'
import {
  mount
} from 'vue-test-utils'
import sinon from 'sinon'
import {
  assert
} from 'chai'

describe('ComponentMixin.js', () => {
  it('should', () => {
    Component.mixins['0']['computed']['property'] = () => true // Considering mixins computed property is has key property

    const wrapper = mount(Component)
    
    assert.equal(wrapper.vm.property, true)
  })
})

Use Vue test utils and mock using options with computed for mixin computed properties and methods for mixin methods.

https://vue-test-utils.vuejs.org/api/options.html#other-options