testlib: add tests for `ConcurrentMap` to make sure its derived `Set`s are concurrent

• 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 https://github.com/ben-manes/caffeine/issues/1883