Draft for str tests.

Tell me what you think.
I agree. ISTM that the "trick" for this task is to come up with some version of `for ds in datashapes` in the code above that

* covers a good chunk of the datashape space

* is fairly easy to extend 

My instinct is that some kind of composition would make sense. For example, composing all of the primitive types with the dimension types in some way.