New Uize.Test subclasses + Uize.Comm updates + other goodies

This pull request is highlighted by new `Uize.Test` subclasses aimed at providing helper methods for testing `Uize.Class` subclasses, `Uize.Widget` subclasses, and widget bindings.

There are also improvements to `Uize.Comm` and `Uize.Comm.Ajax` for error handling. The AJAX object would die before if an error occurred.

There are other miscellaneous fixes outlined in the commit logs.
Could we make children a derived property here, whose value is derived from the test property?