testlib: add tests for ConcurrentMap to make sure its derived Sets are concurrent #7855

• map.entrySet().stream().toList() should return the same elements
• spliterator().characteristics() should probably align with map/set characteristic. For instance, if the map permits nulls, and there are nulls, then Spliterator.NONNULL does not sound right.

See ben-manes/caffeine#1883