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.

... when the operand overwrites mapping with a key that is == but ne.

This is consistent with the super class implementation, which
was overridden in 2.12.11 for efficiency.

I also found a pair of ClassCastExceptions in the new implementations
of `HashMap.++:`, for instance:

```
case class C(a: Int)(override val toString: String); implicit val Ordering_C: Ordering[C] = Ordering.by(_.a); val c0l = C(0)("l"); val c0r = C(0)("r"); import collection.immutable._; println(HashMap((c0l, ())).++:(TreeMap((c0r, ()))))'; done
v2.12.10
Map(r -> ())
v2.12.11
java.lang.ClassCastException: scala.collection.immutable.HashMap$adder$1$ cannot be cast to scala.collection.immutable.HashMap
	at scala.collection.immutable.HashMap$adder$1$.<init>(HashMap.scala:215)
```