UnitTest ContentType to ensure it works as expected

[carbontwelve]

While working on The TaxonomyArchiveGenerator for #315 I stumbled upon a half complete refactoring of ContentType.

It would appear that back in March 2018 I ContentType was refactored with the idea being that it would store references to SourceInterface within its $items property instead of the uid -> timestamp format that had been used in version one of Tapestry.

This was done so that the Project container wouldn't need to be handed around so much with the content type being able to hand back the relevant SourceInterfaces for getSourceList.

Before closing this will need duplicating for Taxonomy.

[carbontwelve]

Majority of this issue is completed by #323.

[carbontwelve]

I closed this prematurely, Taxonomy needs modifying so it contains reference to the Source's as well.