Add test for _redirects with If-None-Match headers

[lidel]

We need a regression test for ipfs/boxo#412

iiuc it should have _redirects

# Map SPA routes to the main index HTML file.
/* /index.html 200

and make request for /ipfs/cid/MISSING-PAGE twice, second time sending If-None-Matc with Etag value from the first response.

There is a test already on the linked PR, and aCAR fixture which we should reuse as-is:
Ref. https://github.com/ipfs/boxo/pull/412/files#diff-d34c7069ce5e81d45082b19eb3e869ee1a086e185dcd6630e75e3ed0d368b546