The previous commit (9fa9ef89) added support for the new SilverStripe 4
feature of Many Many relationships through an intermediary object. After
much head scratching and community testing, the solution was proven to
work, however had no automated tests to confirm as such. This commit
rectifies that by testing both versioned and unversioned DataObjects in
a many_many through style relationship. Some minor tidy and comments
were also added as per feedback on the functionality code changes.
