How to call a method inside a component?

Question

Vue.js

How to use the testFunction method inside the test-26. v-on:click="testFunction"
The problem is that inside the test-26.vue file it does not see v-on:click="testFunction", which is written in app.js
inside test-26.vue, something like this

<template>
<button @click="testFunction">текст</button>
</template>

when pressed, it complains that testFunction does not exist for it.

Vue.component('test-n', require('./components/tests/test-n.vue'));
Vue.component('test-26', require('./components/tests/test-26.vue'));


Vue.http.headers.common['X-CSRF-TOKEN'] = window.Laravel.csrfToken;

let topicInfo = {
    template_id: '',
    timer: '',
    type: '',
    tasks: '',
    solutions: '',
};
const app = new Vue({
    el: '#app',
    data: {
        currentView: 'test-1',
        step: 1,
        topicInfo,
        name: 'Vue.js'
    },
    methods: {
        prev() {
            this.step--;
        },
        next() {
            this.step++;
        },

        mounted() {
            console.log('Component test-1.');
        },

        onSubmitForm() {

            let info = this.topicInfo;

            this.$http.post('/backend/topics', info).then((response) => {
                console.log('request completed');
            }).catch((e) => {
                console.log(e);
            });

        },

        testFunction: function (event) {
            console.log('test clicked')
        },
  
    },
});