Implement a fixture for create_ds_in_store

[mih]

The situation and rational is similar to #62. This helper from datalad.customremotes.ria_utils is not actually creating a dataset in a store, but it creates an empty location for a particular dataset (given by its UUID).

The standard use pattern seems to be that a locally created dataset provides the UUID, and a manually created store the necessary input.

Until proven otherwise, it seems sensible to implement a fixture that depends on

• Implement fixture for create_store #62
• Implement fixture for a populated test dataset #64

and performs the setup for the store and dataset yielded by these fixtures, and returns all relevant info.

It would be better named dataset_in_riastore