Vue testing question - spy on mixin methods

example_componentWithMixin.js

// Component
export default {
    mixins: [NotificationMixin],
    methods: {
        okNotification() {
            this.showNotification('OK')
        } 
    }
}

// Mixin
export default {
    methods: {
        showNotification(msg) {
            // do notifying
        }
    }
}