[Fuchsia] TestServer rework for new SSHPortForwarder functionality.

Made a minor change to make sure that SetupTestServer interacts
properly with the new _ConnectPortForwardingTask and ForwardingScheme
versions.

Change-Id: I16add737f7595fac416973c1296882f9f40d0dc1
Reviewed-on: https://chromium-review.googlesource.com/1178381Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583900}

build/fuchsia/net_test_server.py

@@ -137,8 +137,8 @@ def SetupTestServer(target, test_concurrency):
   logging.debug('Starting test server.')
   spawning_server = chrome_test_server_spawner.SpawningServer(
       0, SSHPortForwarder(target), test_concurrency)
-  forwarded_port = _ConnectPortForwardingTask(
-      target, spawning_server.server_port)
+  scheme = ForwardingScheme(spawning_server.server_port, 0, True)
+  forwarded_port = _ConnectPortForwardingTask(target, scheme)
   spawning_server.Start()
 
   logging.debug('Test server listening for connections (port=%d)' %