Locally overwrite Behat tests

[Kristina Hoeppner]

When a site has customisations that affect the behaviour of a Mahara site, the Behat tests need to be adjusted. However, currently, there is no way of doing that outside of the normal Behat directory and the core tests would need to be updated. That can easily lead to merge conflicts that need to be resolved.

It would be better if local Behat tests could be kept in a separate directory and overwrite the core Behat tests, similarly to what we do with the local language and theme changes etc. in the /local directory.