Properly reset FlumeBuilder SourceFactory after testing

In com.cloudera.flume.agent.TestFlumeNode#testRestartNextException():

FlumeBuilder.setSourceFactory(new SourceFactory() {
  public EventSource getSource(Context ctx, String name, String... args) {
    return new NextExnSource();
  }
}

This pollutes the JVM for all future tests leading to spurious errors in that it returns NextExnSource for any source that gets constructed.