[Spark-3490] Disable SparkUI for tests

We currently open many ephemeral ports during the tests, and as a result we occasionally can't bind to new ones. This has caused the `DriverSuite` and the `SparkSubmitSuite` to fail intermittently.

By disabling the `SparkUI` when it's not needed, we already cut down on the number of ports opened significantly, on the order of the number of `SparkContexts` ever created. We must keep it enabled for a few tests for the UI itself, however.
is this meant for the user to set?  If so we should document.  If not perhaps we should start naming internal configs internal.  or test. for testing ones.