test: replace flaky large file URLs with local fixture - #2294

I decided to remove the /fragment endpoint again. Fragment behavior is already covered by dedicated tests, so this now keeps only the small raw TCP fixture needed to model an incomplete 10 GB response. wiremock buffers response bodies and delays before sending headers, which cannot represent this case. The listener handles two connections because the test invokes lychee twice against the same URL: once with --include-fragments and no fragment, and once with a fragment but without --include-fragments.

That's a great alternative, thanks for the fix!