Add generative tests at least for API namespace

[jsmassa]

Describe the feature you would like to request

After specs are added with PR #596, we should start using generative tests to ensure the correctness of the specs and the ability of our implementation to deal with all of the input we allow.

Describe the solution you would like

Implementation of additional tests which use the specs to generate input, at least for functions of the API namespace.

Describe alternatives you've considered

1. Using the specs in the tests, which restricts the type of tests that can be done
2. Using the specs in production code, which slows down execution
3. Using the specs during development, which usually has to be triggered manually and with that doesn't ensure correctness of the specs as the code develops.

[TimoKramer]

close @jsmassa ?