Test Proxy Link headers across cache revalidation (#101620)
Adopts #95558. Closes #95558.
Fixes #94945.
Proxy can add alternate-language `Link` headers while `next/font/local`
adds font preloads. PR #100960 already fixes the accumulation reported
in #94945, so this PR retains the regression coverage without the
superseded framework change.
The test checks four alternate links and one font preload across three
confirmed cache regenerations. It runs in production and deploy modes,
reads headers through the standard API, and observes a rendered
timestamp instead of relying on fixed delays.
Co-authored-by: KashapovK <kashapov.konstantin.hr@gmail.com>
