Fuchsia: Add net test server support to the new-style runners.

This CL uses SSH reverse port forwarding to dynamically connect the test
suites running on Fuchsia to the Python-based test servers running on
the controlling machine.

Also adds a logging context to the spawning server library, so that the
verbosity of its logging output can be independently adjusted.

Change-Id: I6099ccc21efdd5e2e6b994e863f8d4b3dadaaae1
Reviewed-on: https://chromium-review.googlesource.com/907633
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535911}
5 files changed