Fix element-plus test failure in `validate` job

This pull request fixes the element-plus test code example. The `validate` job in PR #327 was failed because of [element-plus.js](https://github.com/testing-library/vue-testing-library/blob/main/src/__tests__/element-plus.js). [logs](https://github.com/testing-library/vue-testing-library/actions/runs/17713806036/job/50339579476?pr=327#step:5:151): ```bash [test] FAIL src/__tests__/element-plus.js ● Renders component from third-party library [test] [test] TestingLibraryElementError: Unable to find an accessible element with the role "alert" ``` It seemed that the failure was caused by attempting to get the element with the role "alert" before it was shown. So I fixed this by using `findBy` query to wait for the element to show.