Tests that prefetch gets the UA override without touching sec-ch.

Host: 127.0.0.1:8443
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Sec-Purpose: prefetch
Upgrade-Insecure-Requests: 1
Sec-Speculation-Tags: null
User-Agent: Lynx v0.1
Sec-Fetch-Site: none
Sec-Fetch-Mode: navigate
Sec-Fetch-Dest: document
Referer: https://127.0.0.1:8443/inspector-protocol/prefetch/resources/prefetch-echo-header.html
Accept-Encoding: gzip, deflate, br, zstd
Accept-Language: en-us,en