[SPARK-3751] [mllib] DecisionTree: example update + print options

DecisionTreeRunner functionality additions:
* Allow user to pass in a test dataset
* Do not print full model if the model is too large.

As part of this, modify DecisionTreeModel and RandomForestModel to allow printing less info.  Proposed updates:
* toString: prints model summary
* toDebugString: prints full model (named after RDD.toDebugString)

CC: @mengxr  @chouqin @manishamde @codedeft  Small update (whomever can take a look).  Thanks!
It does not actually fit, but I could rearrange the lines (putting more on the first of the 3 lines) if that is preferred.