[APM] Update unit tests removing 'waitForElement' from @testing-library/react

[cauemarcondes]

After upgrading @testing-library/react some tests are printing warning saying that waitForElement was deprecated. More detail here: https://testing-library.com/docs/dom-testing-library/api-async#waitforelement-deprecated-use-find-queries-or-waitfor

 PASS  plugins/apm/public/components/shared/Links/MachineLearningLinks/MLLink.test.tsx
  ● Console

    console.warn
      `waitForElement` has been deprecated. Use a `find*` query (preferred: https://testing-library.com/docs/dom-testing-library/api-queries#findby) or use `waitFor` instead: https://testing-library.com/docs/dom-testing-library/api-async#waitfor

      77 |   );
      78 | 
    > 79 |   await waitForElement(() => el.container.querySelector('a'));
         |         ^
      80 | 
      81 |   const a = el.container.querySelector('a');
      82 |   return a ? a.getAttribute('href') : '';

      at waitForElement (../node_modules/@testing-library/dom/dist/wait-for-element.js:17:13)
      at getRenderedHref (plugins/apm/public/utils/testHelpers.tsx:79:9)
      at Object.<anonymous>.test (plugins/apm/public/components/shared/Links/MachineLearningLinks/MLLink.test.tsx:13:22)

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[smith]

@cauemarcondes I fixed all of these in #79305.

[cauemarcondes]

thanks @smith, closing it!!