arc/tracing: pull out test base helper code

Make shareable code useful to testing of overview tracing's dbus logic.

Bug: b:331467487
Test: OverviewTracingHandlerTest
Change-Id: Ie119fd4d731e82ccef2417bdc11e576d765e2e56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5526884


Commit-Queue: Matthew DeVore <matvore@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300859}