Update flakey prerender fallback test (#64001)
This test could flake as it's racing the fallback data loading and the
initial assertion checking the initial fallback text so this skips
waiting for hydration to allow checking fallback text faster.
x-ref:
https://github.com/vercel/next.js/actions/runs/8529790447/job/23366315476?pr=64000
Closes NEXT-2985
