Prefetch: Add a regression test that prefetch queue get stuck

This CL adds a regression test where PrefetchService::prefetch_queue_ is
not empty and PrefetchService::active_prefetch_ is std::nullopt, but no
additional prefetch is started.

Bug: 400233773, 390329781
Change-Id: Iaf4865c9aad60305d2c3c07da1c921a559c0e825
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6310958
Commit-Queue: Taiyo Mizuhashi <taiyo@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1428475}