Clarification for testing docs
Improves testing documentation and configuration

* test requirements_base.txt is no longer pinned to a particular (outdated) selenium version
* docs note workarounds for test errors
* docs point to https://github.com/divio/django-cms/issues/3684 for work on future improvement
Suggest using pip install -Ur for test requirements
Merge pull request #3681 from evildmp/testsdocs

Clarification for testing docs