test: clean Content.spec.js

packages/@vuepress/core/__test__/app/Content.spec.js

-import Content from '../../lib/app/components/Content'
-import { mount } from '@vue/test-utils'
-import modeTestRunner from '@vuepress/test-utils/modeTestRunner'
-import getRouter from '@vuepress/test-utils/getRouter'
-
-function test (mode, localVue) {
-  it(`${mode} - add custom class by default.`, () => {
-    const wrapper = mount(Content, {
-      localVue,
-      router: getRouter()
-    })
-    expect(wrapper.contains('.custom')).toBe(true)
-  })
-
-  it(`${mode} - remove custom when custom set to false.`, () => {
-    const wrapper = mount(Content, {
-      // https://vue-test-utils.vuejs.org/api/options.html#context
-      context: {
-        props: {
-          custom: false
-        }
-      },
-      localVue,
-      router: getRouter()
-    })
-    expect(wrapper.contains('.custom')).toBe(false)
-  })
-}
-
-modeTestRunner('Content', test)
-