[ServiceWorker] Wait for redirect race request in Subresource_FetchHandler_Wins_Redirect test

The fetch handler response can complete before the embedded test server
observes the losing race network request. This makes the request-count
assertion timing-dependent.

Pump the run loop until the server records the request before checking
that it was deduplicated, matching the synchronization used by nearby
race network request tests.

This flake was observed in downstream Windows, where the fetch-handler
response completed before the EmbeddedTestServer request monitor was
visible to the test. The test then reported an expected request count of
1 but observed 0.

Bug: 340949948
Change-Id: Ie8eb45420fd04ece24eb9127d63d2a6b01595fb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8413735
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Monica Chintala <monicach@microsoft.com>
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1700067}