Use possibleStatus code in test automation

[tpluscode]

Commented by @alien-mcl HydraCG/Specifications#191 (comment)

These kind of constructs makes it easier to prepare some automated tests. Knowing what a operation can return allows you prepare some assertions. Also human readable docs is something we cannot ignore if we think about adoption with big players. When a project begins, one of the first questions from higher managers is 'Where are some docs I can take a look at'?

We could think about some draft recommendations on how status codes can be used in automatic testing of a Hydra API. This could lead to developing of such tool in the future.