DRTVWR-418: Revamp testrunner to shutdown server Thread at end.

Instead of having testrunner.run()'s caller pass a Thread object on which to
run the caller's server instance's serve_forever() method, just pass the
server instance. testrunner.run() now constructs the Thread. This API change
allows run() to also call shutdown() on the server instance when done, and
then join() the Thread.

The hope is that this will avoid the Python runtime forcing the process
termination code to 1 due to forcibly killing the daemon thread still running
serve_forever().

While at it, eliminate calls to testrunner.freeport() -- just make the runtime
pick a suitable port instead.