[tests] fix brittle next rewrite probe test (#10934)

It seems the intention of this test was to verify that the rewritten
file matches the original file, but it does so by just asserting on
arbitrary script text, which is brittle and broke as a result of
vercel/next.js#56294

The right way to test this would be to fetch the original script and
compare the contents with the rewritten script, but this relies on
`__NEXT_SCRIPT__` magic which is only available in probe checks. For now
this just asserts on a valid status code, as a chunk that isn't properly
rewritten should 404 in this case.

.changeset/twelve-pots-whisper.md

@@ -0,0 +1,2 @@
+---
+---

packages/next/test/fixtures/23-custom-routes-verbose/vercel.json

@@ -171,7 +171,7 @@
     // should match /_next file after rewrite
     {
       "path": "/hidden/_next/__NEXT_SCRIPT__(/hello)",
-      "mustContain": "createElement"
+      "status": 200
     },
     // should allow redirecting to external resource
     {