Replace WaitForExpect in Testing docs

[connorshea]

Essentially, the most recent versions of Vue CLI/Jest don't play nicely with WaitForExpect anymore, unfortunately. Because of this, I had to switch to waitFor() from the @vue/testing-library package. Only once I did that was I able to upgrade to Jest v27. The "Writing Tests" docs recommend the waitForExpect package, but it might be a good idea to update that to recommend testing library's waitFor, or at least mention it as an alternative? https://villus.logaretm.com/guide/testing#writing-tests

I have a commit here with some more details and the specific change I ended up going with for my own project: connorshea/vglist-frontend@6aa9128

[logaretm]

So I upgraded Jest's dependencies to 28 and the tests seem to pass even with wait-for-expect. I will investigate it more if I can.