Unflake "streaming responses cancel inner stream after disconnect" test

[flakiness metrics](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.suite%3A%22streaming%20responses%20cancel%20inner%20stream%20after%20disconnect%22&agg_m=count&agg_m_source=base&agg_t=count&currentTab=overview&eventStack=&fromUser=false&index=citest&start=1720903715972&end=1728679715972&paused=false)

The flakiness of the test is underreported because due to masked errors,
the test sometimes yields false-positive results.

Due to a slight increase in compilation times for route handlers in #70897,
the test started to fail consistently in that PR.

This PR fixes the flakiness by handling the case where the request might
already be aborted before the response was sent. This leads to the
stream not being consumed, and subsequently its `finished` promise is
never resolved, which finally leads to test timeouts.