3021: Refactor run_test_server.

3021: Refactor run_test_server.

* Always start API server with both https:// and wss:// enabled. Use
  mocks to test the websocket-not-available case.

* Leave the API server running between tests; just hit reset() when
  needed. Wait until exiting the process to stop the server.

* Do not use rake (tmp:cache:clear, db:test:load, db:fixtures:load) to
  reset the API server's database before starting up a new
  server. Instead, call /database/reset after starting it. (If that
  doesn't work, crash instead of proceeding with undefined results.)

* If a parent process has already started an API server and put its
  endpoint in ARVADOS_TEST_API_HOST, use that instead of starting a
  new one, and leave it running at exit.

* Start servers on random port numbers, so [some] test suites can run
  concurrently with others on a single host (assuming they're
  configured to use different databases).