Create linting for sync mounting in Vue test utils

Soon ™️ the synchronous mode (which is currently the default) will be removed from Vue test utils (https://github.com/vuejs/vue-test-utils/issues/1137). Therefore we should already pass sync: false whenever mounting a component. To enforce this, we could write an ESlint plugin (similar to https://gitlab.com/gitlab-org/frontend/eslint-plugin-i18n).