Add generation of tests

[eseidel]

e.g.

import 'package:test/test.dart';
{{#imports}}
import '{{{.}}}';
{{/imports}}

void main() {
    test(''{{ schemaName }} json round trip'', () {
        final value = {{ schemaName }}.testValue();
        final json = value.toJson();
        final value2 = {{ schemaName }}.fromJson(json);
        expect(value, equals(value2));
    });
}

Will require some sort of "testValue" on the models to generate valid values (even for schemas which don't have defaults).