Improved testing framework for sets and maps.

Switched to JUnit testing framework for sets and maps.  They now test
broadly against each other for consistency.  Tests for mutable.AnyRefMap
and mutable.LongMap are folded in here (originals removed).  There is still
lots of redundancy with other tests that has not been removed.

This framework is also designed to enable more robust testing of changes to
implementations of sets and maps; although it's still quite possible to get
a broken implementation through, these tests should make it harder to get
the fundamentals wrong.
Merge pull request #3306 from Ichoran/topic/junit-set-map

Improved testing framework for sets and maps.