fix flakey app-action test (#63021)
This test is flaking because the assertion before it triggers a redbox
error, and the HMR event to remove the redbox is occasionally happening
after we've moved onto the next test assertion. It also just checks for
"h1" which is heavily overloaded in this test suite for various UI
elements so I added a more specific selector.
[x-ref](https://github.com/vercel/next.js/actions/runs/8195295576/job/22413257207?pr=63019#step:27:1550)
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Closes NEXT-2756
