see https://github.com/maxpou/vue-vuex-jest/blob/master/tests/unit/custom/test-actions.js

testAction.js

export const testAction = (action, payload, state, expectedMutations, done) => {
  let count = 0

  // mock commit
  const commit = (type, payload) => {
    const mutation = expectedMutations[count]

    try {
      expect(mutation.type).toEqual(type)
      if (payload) {
        expect(mutation.payload).toEqual(payload)
      }
    } catch (error) {
      done(error)
    }

    count++
    if (count >= expectedMutations.length) {
      done()
    }
  }

  // call the action with mocked store and arguments
  action({ commit, state }, payload)

  // check if no mutations should have been dispatched
  if (expectedMutations.length === 0) {
    expect(count).toEqual(0)
    done()
  }
}