Add @JSExportDescendentObjects to auto-export all descendent objects.

This annotation, placed on a class or trait, makes all the objects
extending it to be automatically exported, under their fully qualified
name.

This will be useful to export all objects of unit tests, for example.
Use @JSExport features for the testing framework.

This way the testing framework does not relying on the Scala.js
encoding anymore, and is therefore amenable to optimizations through
DCE and/or Closure.