(required_field) option is not covered by code generation

[alexander-yevsyukov]

The option was handled by the runtime validation. It is also covered in a branch of Message.requiresRuntimeValidation() extension in Workaround.kt.

Presumably, requiresRuntimeValidation() returning true should produce the code in the generated validate() method but it does not happen.

Currently, requiresRuntimeValidation() is invoked once in the Validate.violationsOf() method. The violationsOf() method is called in the generated code but it seem to cover only the case of the (validate) option. Please see the following generated stub classes for details:

• io.spine.test.validate.anyfields.AnyContainer
• io.spine.validation.test.MeteoStatistics