Deflake basic.sub.https.html?...&sw=race-fetch-handler-modify-url

- Take `stash.lock` to handle racing requests are counted correctly.
- Add delay to the responses so that both of the racing requests is
  received and counted by the server (before possibly canceled).

Bug: 419222084
Change-Id: I6ee73fa5fce9276810cd1e4262ee0679ef7ee5b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578349
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1464130}