[etw tracing] Emit service logs in tests

When testing the elevated tracing service, use an anonymous pipe to
extract LOG messages from the service and include them in the ordinary
test output.

Bug: 41497361
Change-Id: I76e95714fcff9c6b61477e5d3a757150064e4282
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5929025
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: S Ganesh <ganesh@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369817}