Fix test command

Enable the `--test-port` option. It's a good one to have regardless, but also logic inside `ember-cli`'s `test` command blows up if the option isn't present.

lib/commands/test.js

@@ -38,6 +38,7 @@ function buildAvailableOptions() {
     'environment',
     'config-file',
     'server',
+    'test-port',
     'filter',
     'module',
     'watcher',