[fuchsia] Fix FuchsiaSystemLogging test to timeout quickly on failure.

The test's failure-mode is to timeout, but because it repeatedly ran
new RunLoops for each DumpLogs() call, it would take ~6 minutes to
timeout, and then be terminated by the TestLauncher.

Fix the test to use a single RunLoop, so that it will timeout within
~10 seconds in case of failure.

Bug: 1070686
Change-Id: I054a6b6142835122b2a5f1fee18a189a78392a0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146902
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759268}