[Private Network Access] Rename CorsRfc1918BrowserTest*.

Bug: chromium:1183811
Change-Id: I555dd728c885604368a88ba76a0d15d9278051ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2910329


Auto-Submit: Titouan Rigoudy <titouan@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Titouan Rigoudy <titouan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886223}