[SPARK-3555] Fix UISuite race condition

The test "jetty selects different port under contention" is flaky.

If another process binds to 4040 before the test starts, then the first server we start there will fail, and the subsequent servers we start thereafter may successfully bind to 4040 if it was released between the servers starting. Instead, we should just let Java find a random free port for us and hold onto it for the duration of the test.