[WebAuthn] Mock Windows biometric checks in unit tests

A change that added a biometric availability check on Windows during
device discovery caused some unit test flakiness. Speculatively, this
seems likely to be caused by the platform API call still being in
progress sometimes when the test completes, since `RunUntilIdle` won't
know to wait for it.

By instantiating an override, the API call is circumvented.

Bug: 351529940
Change-Id: I34d8edbcf683e478f56b500ce017dd8735b372ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5677334
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Auto-Submit: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323991}