Remove the use of the TestXXXX classes.

[ewoutkramer]

The TestPatient/TestXXXX classes are becoming a burden since they have not been updated (it's quite a bit of work) and they start to diverge from the actually generated poco's. Since we're going to make more changes in 6.0, I have taken the step to remove them from our source code. Our parsing frameworks are now flexible enough that we can just pick STU3 to test against (the TestXXX classes were derived from STU3 anyway).

[ewoutkramer]

Note that I had to jump through a hoop because it turns out we have two classes in STU3 and Base with exactly the same name (JsonSerializerOptionExtensions). This led to ambiguous references, unless I turned one (internal) function into an extension function. But in 6.0 we might want to rename this anyway to avoid such confusions.