[backport] Bulk ops on TreeSet, TreeMap prefer keys from the left

The test suite that aimed to bring all of our sets and maps into
line did not exercise code paths optimized for:

   collection.$op(collectionOfSameType)

This commit updates the test to:

   - add test variants that generate operands of the same type as the
     input collection
   - add "preserves identity" tests for set intersection
   - fix the resulting failures by flipping the operands
     in `RedBlackTree.{union,intersection}`

I've also added concrete test cases to `Tree{Map,Set}Test` that
are easier to review and that also exercise builders.

Author: retronym

library/src/scala/collection/immutable/RedBlackTree.scala

@@ -760,9 +760,9 @@ private[collection] object RedBlackTree {
   // of child nodes from it. Where possible the black height is used directly instead of deriving the rank from it.
   // Our trees are supposed to have a black root so we always blacken as the last step of union/intersect/difference.
 
-  def union[A, B](t1: Tree[A, B], t2: Tree[A, B])(implicit ordering: Ordering[A]): Tree[A, B] = blacken(_union(t1, t2))
+  def union[A, B](t1: Tree[A, B], t2: Tree[A, B])(implicit ordering: Ordering[A]): Tree[A, B] = blacken(_union(t2, t1))
 
-  def intersect[A, B](t1: Tree[A, B], t2: Tree[A, B])(implicit ordering: Ordering[A]): Tree[A, B] = blacken(_intersect(t1, t2))
+  def intersect[A, B](t1: Tree[A, B], t2: Tree[A, B])(implicit ordering: Ordering[A]): Tree[A, B] = blacken(_intersect(t2, t1))
 
   def difference[A, B](t1: Tree[A, B], t2: Tree[A, _])(implicit ordering: Ordering[A]): Tree[A, B] =
     blacken(_difference(t1, t2.asInstanceOf[Tree[A, B]]))