Automatically generate test data.

This PR enables the `atoum/praspel-extension` in order to automatically generate test data. In this case/PR, since most of the data can be specified by a regular expression (most of the time, written in the code), tests will handle more realistic test data, and therefore, we will be able to detect more bugs or regressions.

The `atoum/praspel-extension` can be used to generate a wide range of data, from regex to grammar, through date, scalar types, objects, arrays etc. This can be useful I think.
not needed as an explicit requirement on a dev version in the root package implies the dev stability flag already