Tests - Update syntax and improve implementation

Update syntax, style, documentation and contracts for the testing suite.

Replace Assigment attempt with object test.
USe across loops instead of traditional loops.
Use a better implementation for converters.
Ok, I don't see it as a test class...
Acording me, in the "new eiffel pratices", a Void result when the parsing is failing is better...

The test in TEST_JSON_CUSTOM_CLASSES is not sufficient?