Unflake `Undefined default export` test (#79632)
[flakiness
metric](https://app.datadoghq.com/ci/test/runs?query=test_level%3Atest%20env%3Aci%20%40git.repository.id%3Agithub.com%2Fvercel%2Fnext.js%20%40test.service%3Anextjs%20%40test.status%3Afail%20%40test.name%3A%22Undefined%20default%20export%20should%20error%20when%20page%20component%20export%20is%20not%20valid%22%20%40git.branch%3Acanary&agg_m=count&agg_m_source=base&agg_t=count¤tTab=overview&eventStack=&fromUser=false&index=citest&start=1746962081472&end=1748258081472&paused=false)
The test's flakiness started to increase significantly on 5/21.
<img width="414" alt="Screenshot 2025-05-26 at 13 18 29"
src="https://github.com/user-attachments/assets/3996cbd1-4a54-412d-9578-317f967030fd"
/>
Using
[these](https://github.com/vercel/next.js/actions/runs/15249566684/job/42888664439?pr=79565)
[two](https://github.com/vercel/next.js/actions/runs/15251316578/job/42888825737?pr=79629)
test runs as examples, we can see that the `/_error` page takes quite a
long time to compile, which then leads to a timeout while waiting for
the `#__next` element on the error page to be present in the DOM.
Looking at the [canary release from this
day](https://github.com/vercel/next.js/releases/tag/v15.4.0-canary.45),
it might be related to #79260.
In fact, running the test before and after this PR uncovers a
significant increase in dev compilation times for the `/_error` page,
which is a pages router page.
<img width="230" alt="Screenshot 2025-05-26 at 13 27 23"
src="https://github.com/user-attachments/assets/00381ab1-3a75-4306-b184-8a14b906d1ff"
/>
We'll address this regression in a follow-up.
